Reverse the ordering of words in a string

Question

I have this string s1    My name is X Y Z  and I want to reverse the order of the words so that s1    Z Y X is name My     I can do it using an additional array  I thought hard but is it possible to do it inplace  without using additional data structures  and with the time complexity being O n

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public class StringReverse      public static void main String   args         StringReverse sr  new StringReverse        String output sr reverse  reverse this string         String substring         for int i 0 i lt  output length   i                      if i  output length                 System out print sr reverse substring                substring              else if output charAt i                      System out print sr reverse substring                    substring                        if i lt output length                      substring  output charAt i                  public String reverse String str       char   value str toCharArray        int count str length        int n   count - 1      for  int j    n-1   gt  gt  1  j  gt   0  --j            char temp   value j           value j    value n - j           value n - j    temp            return new String value

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Here s a nice tweak for those who liked the question    what if the alphabet comprising the swapped words is smaller than 16 chars  16 including  space    there are many examples for such alphabets  the numeric chars  01234567890 -     the genome letters  GATC    the hawaiian alphabet  aeiouhklmnpv    morse code  -     musical notes  ABCDEFG m   etc  This allows encoding the chars as nibbles  4bits  and storing two encoded chars inside one 8bit char   It s still not trivial doing the words swap in a single loop with one cursor moves left-to-right and the other right-to-left  There are actually 4 types of word-copying  copy a word from the left string s side to the right  from the right string s side to the left and the two equivalent copies that involves read write overlapping  position X- X y and X- X-y  where y is smaller than X s length   The nice optimization is that during the first half of the loop words from the right side are encoded into the left  preserving the original left values   but then on the second half words from the left can be copied directly to right and then the left chars are rewritten with their final values     Here s the C code  which takes any alphabet as parameter    define WORDS DELIMITER       define UNMAPPED         0xFF   define BITS IN NIBBLE   4  define BITS IN BYTE     8  define CHARS IN NIBBLE   1  lt  lt  BITS IN NIBBLE   define CHARS IN BYTE     1  lt  lt  BITS IN BYTE   typedef union flip char        unsigned char clear      struct           unsigned char org 4          unsigned char new 4        encoded    flip char t   typedef struct codec        unsigned char nibble2ascii CHARS IN NIBBLE       unsigned char ascii2nibble CHARS IN BYTE     codec t   static int  codec init  const unsigned char  alphabet  codec t  codec    size t len   strlen alphabet   i   if  len  gt  CHARS IN NIBBLE        fprintf stderr   alphabet is too long  n        return -1    if  strchr alphabet  WORDS DELIMITER     NULL        fprintf stderr   missing space in the alphabet n        return -1    strcpy codec- gt nibble2ascii  alphabet   memset codec- gt ascii2nibble   UNMAPPED  CHARS IN BYTE   for  i 0  i lt len  i          codec- gt ascii2nibble  alphabet i      i    return 0     static inline int is legal char  const codec t  codec  const unsigned char ch    return codec- gt ascii2nibble ch     UNMAPPED     static inline unsigned char  encode char  const codec t  codec  unsigned char org  unsigned char new    flip char t flip  flip encoded org   codec- gt ascii2nibble org   flip encoded new   codec- gt ascii2nibble new   return flip clear      static inline unsigned char  decode org  const codec t  codec  unsigned char ch    flip char t flip      clear   ch    return codec- gt nibble2ascii flip encoded org      static inline unsigned char  decode new  const codec t  codec  unsigned char ch    flip char t flip      clear   ch    return codec- gt nibble2ascii flip encoded new         static void inline    encode char  const char  alphabet  const char   static int  flip words  const unsigned char  alphabet  unsigned char  a  size t len    codec t codec         mappings of the 16char-alphabet to a nibble    int r len-1           right reader cursor  moves from right to left scanning for words    int l 0               left writer cursor  moves from left to right    int i 0                          word iterator    int start word -1 end word -1     word boundaries    unsigned char org r 0            original value pointed by the right cursor    int encode 0  rewrite 0          writing states     if  codec init alphabet   amp codec   lt  0  return -1      parse the buffer from its end backward    while  r gt  0        if  r gt  l  amp  amp   is legal char  amp codec  a r              fprintf stderr   illegal char  c in string n   a r            return -1               read the next charachter looking for word boundaries        org r    r lt l    decode org  amp codec  a r     a r          handle word boundaries        if  org r    WORDS DELIMITER               mark start of word  next char after space after non-space              if  end word gt 0  amp  amp  start word lt 0  start word   r  skip this space   1             rewrite this space if necessary  2nd half             if  r lt l  a r    decode new  amp codec a r          else              mark end of word  1st non-space char after spaces             if  end word lt 0  end word   r             left boundary is a word boundary as well            if   r  start word   r               Do we have a complete word to process         if  start word lt 0    end word lt 0            r--          continue               copy the word into its new location        for i start word  i lt  end word  i    l              if  i gt  l  amp  amp   is legal char  amp codec  a l                  fprintf stderr   illegal char  c in string n   a l                return -1                       reading phase  value could be encoded or not according to writer s position            org r   i lt l    decode org  amp codec  a i     a i              overlapping words in shift right  encode and rewrite            encode rewrite  l gt  start word  amp  amp  l lt  end word  amp  amp  i lt l              1st half - encode both org and new vals            encode   start word-1 gt l              2nd half - decode and rewrite final values            rewrite   i lt l              writing phase            a l   encode   encode char  amp codec  a l   org r    org r          if  rewrite                a i  decode new  amp codec  a i                           done with this word         start word end word -1         write a space delimiter  unless we re at the end        if  r            a l    l lt r   encode char  amp codec  a l   WORDS DELIMITER    WORDS DELIMITER          l              r--    a l  0  return 0     All Done

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public static String ReverseString String str        int word length   0      String result           for  int i 0  i lt str Length  i                  if  str i                                result         result              word length   0            else                        result   result Insert word length  str i  ToString                 word length                        return result      This is C  code

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We can insert the string in a stack and when we extract the words  they will be in reverse order   void ReverseWords char Arr          std  stack lt std  string gt  s      char  str      int length   strlen Arr       str   new char length 1       std  string ReversedArr      str   strtok Arr           while str   NULL                s push str           str   strtok NULL                 while  s empty                  ReversedArr   s top            cout  lt  lt       lt  lt  ReversedArr          s pop

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Better version Check my blog http   bamaracoulibaly blogspot co uk 2012 04 19-reverse-order-of-words-in-text html  public string reverseTheWords string description        if   string IsNullOrEmpty description    amp  amp   description IndexOf       gt  1                 string   words  description Split               Array Reverse words           foreach  string word in words                        string phrase   string Join      words               Console WriteLine phrase                     return phrase            return description

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This is assuming all words are separated by spaces    include  lt stdio h gt   include  lt string h gt   int main         char string      What are you looking at       int i  n   strlen string        int tail   n-1      for i n-1 i gt  0 i--                if string i            i    0                        int cursor    i  0  i  i 1               while cursor  lt   tail                  printf   c   string cursor                  printf                   tail   i-1                      return 0

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By doing a rotation of the string you can reverse the order of the words without reversing the order of the letters  As each word is rotated to the end of the string we reduce the amount of the string to be rotated by the length of the word   We then rotate any spaces to in front of the word we just placed at the end and shorten the rotating part of the string by 1 for each space   If the entire string is rotated with out seeing a space we are done  Here is am implementation in C    The l variable contains the length of the buf to rotate  and n counts the number of rotations made       include  lt stdio h gt       include  lt string h gt           int main char  argv    int argc              char buf 20     quot My name is X Y Z quot         int l   strlen buf         int n   1             while  l  gt   n            unsigned char c   buf 0           for  int i   1  i  lt  l  i                buf i-1    buf i                     buf l - 1    c          if  buf 0            c                     l -  n            n   1            else             n                                 printf  quot  s n quot   buf          The rotation code was purposely left very simple

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My version of using stack   public class Solution       public String reverseWords String s            StringBuilder sb   new StringBuilder            String ns  s trim            Stack lt Character gt  reverse   new Stack lt Character gt             boolean hadspace false             first pass         for  int i 0  i lt  ns length   i                 char c   ns charAt i               if  c                        if   hadspace                       reverse push c                       hadspace true                                 else                  hadspace false                  reverse push c                                   Stack lt Character gt  t   new Stack lt Character gt             while   reverse empty                 char temp  reverse pop                if temp                          get the stack content out append to StringBuilder                 while   t empty                         char c  t pop                        sb append c                                     sb append                    else                    push to stack                 t push temp                                   while   t empty                 char c  t pop                sb append c                     return sb toString

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public static void main String args          String str    My name is X Y Z      out put  Z Y X is name My          split     String   arr   str split              reverse word     String reverse           for  int i   arr length - 1  i  gt   0  i--            if i  0               reverse    arr i                else              reverse    arr i                        System out println reverse

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Reverse the entire string  then reverse the letters of each individual word   After the first pass the string will be  s1    Z Y X si eman yM    and after the second pass it will be  s1    Z Y X is name My

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In c  this is how you might do it  O N  and only using O 1  data structures  i e  a char     include lt stdio h gt   include lt stdlib h gt  main      char  a   malloc 1000     fscanf stdin       0 n    a     int x   0  y    while a x     0           if  a x          a x     n               x              else             y x        while a y     0   amp  amp  a y        amp  amp  a y     n                    y                  int z y        while x lt y                  y--          char c a x  a x  a y  a y  c           x                  x z               fprintf stdout a     return 0

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You cannot do the reversal without at least some extra data structure   I think the smallest structure would be a single character as a buffer while you swap letters   It can still be considered  in place   but it s not completely  extra data structure free    Below is code implementing what Bill the Lizard describes   string words    this is a test       Reverse the entire string for int i   0  i  lt  strlen words    2    i      char temp   words i     words i    words strlen words  - i     words strlen words  - i    temp        Reverse each word for int i   0  i  lt  strlen words     i      int wordstart   -1    int wordend   -1    if words i                wordstart   i      for int j   wordstart  j  lt  strlen words     j          if words j                    wordend   j - 1          break                    if wordend    -1        wordend   strlen words       for int j   wordstart   j  lt    wordend   wordstart    2     j          char temp   words j         words j    words wordend -  j - wordstart          words wordend -  j - wordstart     temp            i   wordend

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Not exactly in place  but anyway  Python    gt  gt  gt  a    These pretzels are making me thirsty   gt  gt  gt      join a split     -1    thirsty me making are pretzels These

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This Question is asked in Paytm interview for Java position  I come up with the following solution   class ReverseStringWord  public static void main String   args        String s  My name is X Y Z       StringBuilder result new StringBuilder        StringBuilder str new StringBuilder        for int i 0 i lt s length   i             if s charAt i                     result insert 0 str                   str setLength 0                     else              str append s charAt i                if i  s length  -1                   result insert 0 str                                         System out println result

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What language  If PHP  you can explode on space  then pass the result to array reverse   If its not PHP  you ll have to do something slightly more complex like   words   aString split       for  i   0  i  lt  words length  i          words i    words words length-i

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string    hello world   strrev      list       def splitstring string       j   0      for i in range 0 len string            if string i                      list append string j i               j   i 1          elif  i 1    len string                list append string j i 1     splitstring string   for i in list      for j in range len i -1 -1 -1           strrev    i j       if  i    list -1            strrev        print list    print    s     strrev

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I know there are several correct answers  Here is the one in C that I came up with  This is an implementation of the excepted answer  Time complexity is O n  and no extra string is used    include lt stdio h gt   char   strRev char  str  char tok       int len   0  i     char  temp   str     char swap      while  temp    tok  amp  amp   temp      0           len    temp               len--      for i   0  i  lt  len 2  i            swap   str i         str i    str len - i          str len - i    swap                 Return pointer to the next token     return str   len   1     int main void       char a      Reverse this string       char  temp   a      if  a    NULL        return -1          Reverse whole string character by character     strRev a    0           Reverse every word in the string again     while 1          temp   strRev temp              if   temp      0            break         temp               printf  Reversed string   s n   a       return 0

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How about      var words    My name is X Y Z   var wr   String Join       words Split      Reverse   ToArray        I guess that s not in-line tho

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c  solution to reverse words in a sentence  using System  class helloworld       public void ReverseString String   words            int end   words Length-1          for  int start   0  start  lt  end  start                  String tempc              if  start  lt  end                     tempc   words start                   words start    words end                   words end--    tempc                                  foreach  String s1 in words                Console Write   0    s1                     class reverse       static void Main             string s   beauty lies in the heart of the peaople           String   sent char s Split               helloworld h1   new helloworld            h1 ReverseString sent char             output  peaople the of heart the in lies beauty Press any key to continue

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public class reversewords    public static void main String args        String s  my name is nimit goyal        char a   s toCharArray        int x s length        int count 0      for int i s length  -1  i gt -1 i--                 if a i         i  0              System out print  hello                if i  0               System out print                    for int k i k lt x k                                  System out print a k                    count                              x i            count               System out println          System out println  total run     count          output  goyal nimit is name  my  total run   46

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This is not perfect but it works for me right now  I don t know if it has O n  running time btw  still studying it     but it uses one additional array to fulfill the task   It is probably not the best answer to your problem because i use a dest string to save the reversed version instead of replacing each words in the source string  The problem is that i use a local stack variable named buf to copy all the words in and i can not copy but into the source string as this would lead to a crash if the source string is const char   type   But it was my first attempt to write s th  like this    Ok enough blablub  here is code    include  lt iostream gt  using namespace std   void reverse char  des  char   const s   int main  int argc  const char   argv              char  s    char   reservered  rights All Saints  The 2011  c  Copyright 11 10 11 on Pfundstein Markus by Created       char  x    char   Dogfish  White-spotted Shark  Bullhead        printf  Before    s  n   x       printf  Before    s  n   s        char  d    char  malloc  strlen s  1  sizeof char          char  i    char  malloc  strlen x  1  sizeof char         reverse d s       reverse i x        printf  After    s  n   i       printf  After    s  n   d        free  i       free  d        return 0     void reverse char  dest  char  const s           create a temporary pointer     if  strlen s   0  return      unsigned long offset   strlen s  1       char  buf    char  malloc  offset  sizeof char        memset buf  0  offset        char  p         iterate from end to begin and count how much words we have     for  unsigned long i   offset  i    0  i--            p   s i             if we discover a whitespace we know that we have a whole word         if   p            p      0                    we increment the counter             if   p      0                        we write the word into the buffer                   p                  int d    int  strlen p -strlen buf                    strncat buf  p  d                   strcat buf                                              copy the last word     p -  1      int d    int  strlen p -strlen buf        strncat buf  p  d       strcat buf    0            copy stuff to destination string     for  int i   0  i  lt  offset    i              dest i    buf i              free buf

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This quick program works  not checks the corner cases though     include  lt stdio h gt   include  lt stdlib h gt  struct node       char word 50       struct node  next     struct stack       struct node  top     void print  struct stack  stk   void func  struct stack   stk  char  str   main         struct stack  stk   NULL      char string 500     the sun is yellow and the sky is blue       printf   n s n   string       func   amp stk  string       print  stk     void func  struct stack   stk  char  str        char  p1   str      struct node  new   NULL   list   NULL      int i  j      if   stk    NULL                 stk    struct stack  malloc sizeof struct stack            if   stk    NULL              printf   n        stack is not allocated       n              stk - gt top   NULL            i   0      while    p1 i       0                 if    p1 i                                new    struct node  malloc sizeof struct node                if  new    NULL                  printf   n        new is not allocated       n                j   0              while    p1 i          amp  amp    p1 i       0                                 new- gt word j      p1   i                   i                    j                              new- gt word j                       new- gt word j      0               new- gt next     stk - gt top                stk - gt top   new                    i           void print  struct stack  stk        struct node  tmp   stk- gt top      int i      while  tmp    NULL                i   0          while  tmp- gt word i       0                         printf    c    tmp- gt word i                i                      tmp   tmp- gt next            printf   n

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Here is a C implementation that is doing the word reversing inlace  and it has O n  complexity   char  reverse char  str  char wordend 0        char c      size t len   0      if  wordend  0            len   strlen str             else           for size t i 0 str i   wordend  amp  amp  str i   0 i                len   i 1                    for size t i 0 i lt len 2 i                      c   str i                   str i    str len-i-1                   str len-i-1    c                    return str     char  inplace reverse words char  w        reverse w      reverse all letters first     bool is word start    w 0   0x20        for size t i 0 i lt strlen w  i             if w i   0x20  amp  amp  is word start                reverse  amp w i   0x20      reverse one word only             is word start   false                    if   is word start  amp  amp  w i   0x20     found new word             is word start   true            return w

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Store Each word as a string in array then print from end  public void rev2         String str    my name is ABCD       String A     str split            for  int i   A length - 1  i  gt   0  i--            if  i    0                System out print A i                    else               System out print A i

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Usage  char str 50     0   strcpy str   char   My name is Khan    reverseWords str     Method  void reverseWords char  pString       if NULL   pString           return            int nLen       strlen pString       reverseString pString nLen       char  start    pString      char  end      pString      nLen           0      while   end            if  end                       reverseString start nLen               end                start   end              nLen    0              continue                    nLen            end              reverseString start nLen       printf   n Reversed   s  pString        void reverseString char  start int nLen       char  end   start  nLen-1      while nLen  gt  0           char temp    start           start       end           end        temp          end--          start            nLen- 2

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This is how you solve it in TCL no matter how many spaces or tabs or new lines   n  characters exist in your string  this is a real life solution and the human way of thinking  It does not consider one and only one space is a mark of a new  word    And i think in C C   and Java you can translate it the same way   I worked on it for two days  before this posting   because i did not accept that there exist functions provided by the library of the language which are also made for us  and we don t use them    lt  -- language  lang-php -- gt    1- Reverse the orignial text set reversed   string reverse   all original text      2- split the reversed string  reversed into a list of words then loop over them set list of reversed words  split  reversed     foreach reversed words  list of reversed words      3- find the indices of the extremes of each reversed word in the  reversed  set word start   string first  reversed words  reversed  word start  set word end   expr  word start -1    string length  letter       4- reverse the current-in-the-loop reversed word back to its normal state  e g     if i have a word  loohcs  then convert it by reversing it to  school   set original word  string reverse   string range  reversed  word start  word end       5- replace  the reversed word  loohcs  with the correcte one  school   set reversed   string replace  reversed  word start  word end  original word     6- set the start-of-search index to the index    directly after the ending of the current word  set word start  expr  word end  1     7-continue to the next loop       print the result puts  finally   reversed

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reverse the string and then  in a second pass  reverse each word     in c   completely in-place without additional arrays   static char   ReverseAllWords char   in text        int lindex   0      int rindex   in text Length - 1      if  rindex  gt  1                  reverse complete phrase         in text   ReverseString in text  0  rindex              reverse each word in resultant reversed phrase         for  rindex   0  rindex  lt   in text Length  rindex                          if  rindex    in text Length    in text rindex                                        in text   ReverseString in text  lindex  rindex - 1                   lindex   rindex   1                                    return in text     static char   ReverseString char   intext  int lindex  int rindex        char tempc      while  lindex  lt  rindex                tempc   intext lindex           intext lindex      intext rindex           intext rindex--    tempc            return intext

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Using Java        String newString       String a    My name is X Y Z       int n   a length        int k   n-1      int j 0       for  int i n-1  i gt  0  i--                if  a charAt i            i  0                         j   i  0  i 1 i               while j lt  k                                newString   newString   a charAt j                   j j 1                            newString   newString                    k i-1                       System out println newString     Complexity is O n   traversing entire array    O n   traversing each word again    O n

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using System  using System Collections Generic  using System Linq  using System Text  using System Threading Tasks   namespace ReverseString       class Program               static void Main string   args                        string StringReverse                   int StringLength              do                               Console WriteLine  Enter the String                       string InputString   Console ReadLine                     Console WriteLine  Enter the length                       int InputLength   Convert ToInt32 Console ReadLine                      int NewLength   InputLength                  InputLength   InputLength - 1                  int LengthString   InputString Length                  int l   LengthString - NewLength                  StringReverse                        while  InputLength  gt   0                                        StringReverse   StringReverse   InputString InputLength                       InputLength--                                     String substr   InputString Substring NewLength  l                    Console WriteLine  Reverse  String  Is   0    StringReverse   substr                    Console WriteLine   tDo you Want to CONTINUE  n t1 YES n t2 NO                    StringLength   Convert ToInt32 Console ReadLine                                while  StringLength    1

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Printing words in reverse order of a given statement using C        void ReverseWords string str                int j   0          for  int i    str Length - 1   i  gt   0  i--                        if  str i            i    0                                j   i    0   i   i   1                   while  j  lt  str Length  amp  amp  str j                              Console Write str j                      Console Write

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I did solve the problem without using extra space used only the original string itself  but I couldn t solve the problem in O n   the least I could get is O n square  that s for the worst case scenario    The way how I implemented -   Reverse the whole sentence  char by char   Later iterate the whole sentence but this time reverse each word    AND THAT S WHY I GOT THE WORST TIME COMPLEXITY AS O n sqaure   Please find the code below in java  hope it helps someone    class MainClass       public static void main String args               String str    reverse me  also lets check           System out println  The initial string is  --- gt  gt      str            for  int i   0  i  lt  str length    i                    Keep iterating each letter from one end to another              str   str substring 1  str length   - i                    str substring 0  1                    str substring str length   - i  str length                       System out println  The reversed string is ---- gt  gt      str            for  int i   0  j   0  i  lt  str length    i                  if str charAt i            i     str length   - 1                       Just to know the start of each word                 int k   j                     the below if condition is for considering the last word                  if i     str length   - 1   i                     while j  lt  i                        j                        str   str substring 0  k                             i-j  is the length of each word                           str substring k   1   k   1    i - j                            str substring k  k   1                            str substring  k   1    i - j  i                            str substring i                       if j    i                              Extra j   for considering the empty white space                         j                                                                            System out println  The reversed string but not the reversed words is ---- gt  gt      str

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The simplest way to do this              string inputStr    My name is X Y Z             string outputStr   string Empty            List lt string gt  templist1   new List lt string gt               templist1   inputStr Split      ToList               for  int i   templist1 Count- 1   i  gt   0  i--                            if  outputStr    string Empty                                    outputStr   outputStr         templist1 i                                 else                                   outputStr   templist1 i                                           Console WriteLine  Reverse of a String is   outputStr            Output          Z Y X is name My

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class Program       static void Main string   args                string s1    My Name varma           string   arr   s1 Split               Array Reverse arr           string str   string Join      arr           Console WriteLine str           Console ReadLine

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It can be done more simple using sscanf   void revertWords char  s   void revertString char  s  int start  int n   void revertWordsInString char  s    void revertString char  s  int start  int end         while start lt end                  char temp   s start            s start    s end            s end  temp           start             end --             void revertWords char  s      int start   0     char  temp    char   malloc strlen s    1     int numCharacters   0    while sscanf  amp s start     s   temp    EOF            numCharacters   strlen temp          revertString s  start  start numCharacters -1         start   start numCharacters   1        if s start-1     0        return         free  temp       void revertWordsInString char  s       revertString s 0  strlen s -1      revertWords s      int main        char  s  new char  strlen  abc deff gh1 jkl   1      strcpy s  abc deff gh1 jkl       revertWordsInString s      printf   s  s      return 0

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public class manip   public static char   rev char   a int left int right        char temp      for  int i 0 i lt  right - left  2 i                 temp   a i   left           a i   left    a right -i -1           a right -i -1    temp             return a    public static void main String   args  throws IOException        String s   i think this works       char   str   s toCharArray               int i 0      rev str i s length         int j 0      while j  lt  str length            if  str j          amp  amp  j    str length -1                j              else                       if  j     str length -1                       j                              rev str i j               i j 1              j i                      System out println str

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This is how to do it in Excel as UDF and in VBA   Public Function ReverseMe textToReverse As String              Optional delim As String        As String      Dim test    As String     Dim arr     As Variant     Dim arr2    As Variant     Dim arrPart As Variant     Dim cnt     As Long      arr   Split textToReverse           ReDim arr2 UBound arr        For Each arrPart In arr         arr2 cnt    StrReverse arrPart          cnt   cnt   1     Next arrPart      ReverseMe   StrReverse Join arr2         End Function  Public Sub TestMe        Debug Print ReverseMe  veso dosev diri rid            Debug Print ReverseMe  VBA is the best language    End Sub

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In Python  if you can t use    -1  or reversed    here is the simple way   def reverse text      r text   text split        res        for word in range len r text  - 1  -1  -1        res append r text word      return     join res   print  reverse  Hello World      gt  gt  World Hello  Finished in 0 1s

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include  lt stdio h gt   include  lt stdlib h gt   include  lt strings h gt   void reverse char  data  int len        int counter   0      int end   len - 1      char temp       for  counter   0  counter  lt  len   2  counter    end--            temp   data counter           data counter    data end           data end    temp           int main void        char data      This is a line and needs to be reverse by words        int c   0      int len   strlen data       int wl   0      int start   0      printf   n    data     s   data       reverse data  len        for  c   0  c  lt  len  c              if   wl                start   c                    if  data c                        wl              else               reverse data   start  wl               wl   0                        printf   nnow data     s   data       return EXIT SUCCESS

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Most of these answers fail to account for leading and or trailing spaces in the input string   Consider the case of str     Hello world      The simple algo of reversing the whole string and reversing individual words winds up flipping delimiters resulting in f str      world Hello       The OP said  I want to reverse the order of the words  and did not mention that leading and trailing spaces should also be flipped   So  although there are a ton of answers already  I ll provide a  hopefully  more correct one in C      include  lt string gt   include  lt algorithm gt   void strReverseWords inPlace std  string  amp str      const char delim          std  string  iterator w begin  w end    if  str size      0      return     w begin   str begin      w end     str begin       while  w begin    str end          if  w end    str end       w end    delim          if  w begin    w end          std  reverse w begin  w end         if  w end    str end            break        else         w begin     w end        else           w end                  instead of reversing str begin   to str end    use two iterators that         represent the  logical  begin and end  ignoring leading traling delims   std  string  iterator str begin   str begin    str end   str end      while  str begin    str end  amp  amp   str begin    delim        str begin    --str end    while  str end    str begin  amp  amp   str end    delim      --str end      str end    std  reverse str begin  str end

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In Smalltalk    These pretzels are making me thirsty  subStrings reduce    a  b  b       a    I know noone cares about Smalltalk  but it s so beautiful to me

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Actually  the first answer   words   aString split       for  i   0  i  lt  words length  i          words i    words words length-i       does not work because it undoes in the second half of the loop the work it did in the first half  So  i  lt  words length 2 would work  but a clearer example is this   words   aString split          make up a list i   0  j   words length - 1     find the first and last elements while  i  lt  j        temp   words i   words i    words j   words j    temp    i e  swap the elements     i         j--      Note  I am not familiar with the PHP syntax  and I have guessed incrementer and decrementer syntax since it seems to be similar to Perl

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Here is the algorithm    Explode the string with space and make an array of words  Reverse the array  Implode the array by space

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In Java using an additional String  with StringBuilder    public static final String reverseWordsWithAdditionalStorage String string        StringBuilder builder   new StringBuilder         char c   0      int index   0      int last   string length        int length   string length  -1      StringBuilder temp   new StringBuilder        for  int i length  i gt  0  i--            c   string charAt i           if  c    SPACE    i  0                index    i  0  0 i 1              temp append string substring index  last                if  index  0  temp append c               builder append temp               temp delete 0  temp length                 last   i                       return builder toString        In Java in-place   public static final String reverseWordsInPlace String string        char   chars   string toCharArray         int lengthI   0      int lastI   0      int lengthJ   0      int lastJ   chars length-1       int i   0      char iChar   0      char jChar   0      while  i lt chars length  amp  amp  i lt  lastJ            iChar   chars i           if  iChar    SPACE                lengthI   i-lastI              for  int j lastJ  j gt  i  j--                    jChar   chars j                   if  jChar    SPACE                        lengthJ   lastJ-j                      swapWords lastI  i-1  j 1  lastJ  chars                       lastJ   lastJ-lengthI-1                      break                                              lastI   lastI lengthJ 1              i   lastI            else               i                         return String valueOf chars      private static final void swapWords int startA  int endA  int startB  int endB  char   array        int lengthA   endA-startA 1      int lengthB   endB-startB 1       int length   lengthA      if  lengthA gt lengthB  length   lengthB       int indexA   0      int indexB   0      char c   0      for  int i 0  i lt length  i              indexA   startA i          indexB   startB i           c   array indexB           array indexB    array indexA           array indexA    c             if  lengthB gt lengthA            length   lengthB-lengthA          int end   0          for  int i 0  i lt length  i                  end   endB-  length-1 -i               c   array end               shiftRight endA i end array               array endA 1 i    c                  else if  lengthA gt lengthB            length   lengthA-lengthB          for  int i 0  i lt length  i                  c   array endA               shiftLeft endA endB array               array endB i    c                     private static final void shiftRight int start  int end  char   array        for  int i end  i gt start  i--            array i    array i-1            private static final void shiftLeft int start  int end  char   array        for  int i start  i lt end  i              array i    array i 1

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In Python     ip    My name is X Y Z  words   ip split   words reverse   print     join words    Anyway cookamunga provided good inline solution using python

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import java util Scanner   public class revString      static char   str      public static void main String   args          Initialize string       str   new char      h    e    l    l    o         a         w    o          r    l    d         getInput            reverse entire string     reverse 0  str length - 1           reverse the words  delimeted by space  back to normal     int i   0  j   0      while  j  lt  str length             if  str j            j    str length - 1                 int m   i              int n                 dont include space in the swap                  special case is end of line              if  j    str length - 1                  n   j              else                 n   j -1                  reuse reverse             reverse m  n                i   j   1                     j               displayArray       private static void reverse int i  int j         while  i  lt  j             char temp          temp   str i           str i    str j           str j    temp           i            j--          private static void getInput         System out print  Enter string to reverse          Scanner scan   new Scanner System in       str   scan nextLine   trim   toCharArray        private static void displayArray           Print the array     for  int i   0  i  lt  str length  i              System out print str i

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Here is the Java Implementation   public static String reverseAllWords String given string        if given string    null    given string isBlank            return given string       char   str   given string toCharArray        int start   0          Reverse the entire string     reverseString str  start  given string length   - 1           Reverse the letters of each individual word     for int end   0  end  lt   given string length    end                  if end    given string length      str end                                reverseString str  start  end-1               start   end   1                      return new String str         In-place reverse string method public static void reverseString char   str  int start  int end        while start  lt  end                char temp   str start           str start      str end           str end--    temp

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In JAVA  package Test   public class test2        public static void main String   args            String str    my name is fawad X Y Z            String strf               String strfinal              if  str                  for  int i 0   i lt  str length  -1  i                  strf    str charAt str length   -  i 1                       System out println strf                     else System out println  String is Null                 if  strf                         String   temp   strf split                            String temp1                        System out println temp length                    for  int j 0  j lt  temp length-1  j                          temp1   temp j                        if temp1 length   gt 1                        for  int k 0  k lt  temp1 length  -1  k                              strfinal    temp1 charAt temp1 length  - 1 k                                               strfinal                                                   else strfinal    temp1                                           System out println strfinal                              else System out println  String Final is Null                      Output   Z Y X dawaf si eman ym  Z Y X fawad is name my

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One way to do this is to parse each word of our input string and push it into a LIFO stack    After whole string is processed  we pop out each word off the stack one by one and append it to a StringBuffer class object  which finally contains the reversed input string   This is one possible solution in Java using StringTokenizer and Stack class  We need to import java util Stack   public String revString String input       StringTokenizer words new StringTokenizer input     Split the string into words    Stack lt String gt  stack  new Stack lt String gt        while words hasMoreTokens               stack push words nextElement   toString        Push each word of the string onto stack           StringBuilder revString new StringBuilder       while  stack empty                revString append stack pop           pop the top item and append it to revString          return revString toString

[algorithm] Reverse the ordering of words in a string

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