Check string for palindrome

Question

A palindrome is a word  phrase  number or other sequence of units that can be read the same way in either direction   To check whether a word is a palindrome I get the char array of the word and compare the chars  I tested it and it seems to work  However I want to know if it is right or if there is something to improve    Here is my code   public class Aufg1       public static void main String   args            String wort    reliefpfpfeiller           char   warray   wort toCharArray             System out println istPalindrom warray                      public static boolean istPalindrom char   wort           boolean palindrom   false          if wort length 2    0               for int i   0  i  lt  wort length 2-1  i                     if wort i     wort wort length-i-1                        return false                   else                      palindrom   true                                           else              for int i   0  i  lt   wort length-1  2-1  i                     if wort i     wort wort length-i-1                        return false                   else                      palindrom   true                                                    return palindrom

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public static boolean isPalindrome String word        String str           for  int i word length  -1  i gt  0   i--           str   str   word charAt i            if str equalsIgnoreCase word           return true      else         return false

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Considering not letters in the words  public static boolean palindromeWords String s             int left 0          int right s length  -1           while left lt  right                while left lt right  amp  amp   Character isLetter s charAt left                     left                              while right gt 0  amp  amp   Character isLetter s charAt right                     right--                             if  s charAt left       s charAt right--                     return false                                  return true                      Test public void testPalindromeWords        assertTrue StringExercise palindromeWords  ece         assertTrue StringExercise palindromeWords  kavak         assertFalse StringExercise palindromeWords  kavakdf         assertTrue StringExercise palindromeWords  akka         assertTrue StringExercise palindromeWords    e  c --e

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IMO  the recursive way is the simplest and clearest   public static boolean isPal String s           if s length      0    s length      1          return true       if s charAt 0     s charAt s length  -1          return isPal s substring 1  s length  -1                       return false

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public boolean isPalindrome String abc       if abc    null  amp  amp  abc length    gt  0           char   arr   abc toCharArray            for  int i   0  i  lt  arr length 2  i                  if arr i     arr arr length - 1 - i                    return false                                  return true            return false

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We can reduce the loop to half of the length  function isPallindrome s      let word  s toLowerCase      let length   word length -1    let isPallindrome  true    for let i 0  i lt  length 2  i         if word i      word length -i          isPallindrome  false        break              return isPallindrome

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For-loop contains sub length     2 - 1   It has to be subtracted with 1 as the element in the middle of the string does not have to checked    For example  if we have to check an string with 7 chars  1234567   then 7 2    3 and then we subtrack 1  and so the positions in the string will become  0123456   The chars checked with be the 0  1  2 element with the 6  5  4 respectively  We do not care about the element at the position 3 as it is in the exact middle of the string      private boolean isPalindromic String sub            for  int i   0  i  lt   sub length     2 - 1  i                  if  sub charAt i     sub charAt sub length   - 1 - i                     return false                                  return true

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Using stack  it can be done like this  import java io    import java util    import java text    import java math    import java util regex    import java util     public class Solution        public static void main String   args            Scanner in   new Scanner System in           String str in nextLine            str replaceAll    s                  System out println str           Stack lt String gt  stack new Stack lt String gt             stack push str           String str rev stack pop            if str equals str rev                System out println  Palindrome              else               System out println  Not Palindrome

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also a different looking solution   public static boolean isPalindrome String s             for  int i 0   j s length  -1   i lt j   i     j--                  if   s charAt i     s charAt j                      return false                                   return true

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Recently I wrote a palindrome program which doesn t use StringBuilder  A late answer but this might come in handy to some people   public boolean isPalindrome String value        boolean isPalindrome   true      for  int i   0   j   value length   - 1   i  lt  j   i      j --            if  value charAt i     value charAt j                 isPalindrome   false                      return isPalindrome

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I worked on a solution for a question that was marked as duplicate of this one  Might as well throw it here     The question requested a single line to solve this  and I took it more as the literary palindrome - so spaces  punctuation and upper lower case can throw off the result   Here s the ugly solution with a small test class   public class Palindrome      public static boolean isPalendrome String arg             return arg replaceAll    A-Za-z        equalsIgnoreCase new StringBuilder arg  reverse   toString   replaceAll    A-Za-z                  public static void main String   args          System out println isPalendrome  hiya           System out println isPalendrome  star buttons not tub rats           System out println isPalendrome  stab nail at ill Italian bats            return           Sorry that it is kind of nasty - but the other question specified a one-liner

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Check whether a word is a palindrome        param word the word     param low  low index     param high high index     return   code true  if the word is a palindrome       code false  otherwise     private static boolean isPalindrome char   word  int low  int high        if  low  gt   high            return true        else if  word low     word high             return false        else           return isPalindrome word  low   1  high - 1                   Check whether a word is a palindrome        param the word     return   code true  if the word is a palindrome      code false  otherwise     private static boolean isPalindrome char   word        int length   word length      for  int i   0  i  lt   length   2  i              if  word i     word length - 1 - i                 return false                      return true     public static void main String   args        char   word     a    b    c    b    a         System out println isPalindrome word  0  word length - 1        System out println isPalindrome word

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You can check if a string is a palindrome by comparing it to the reverse of itself   public static boolean isPalindrome String str        return str equals new StringBuilder str  reverse   toString         or for versions of Java earlier than 1 5   public static boolean isPalindrome String str        return str equals new StringBuffer   append str  reverse   toString         EDIT   FernandoPelliccioni provided a very thorough analysis of the efficiency  or lack thereof  of this solution  both in terms of time and space  If you re interested in the computational complexity of this and other possible solutions to this question  please read it

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In PHP  function isPalindrome  string        return  strrev  string      string    true   false     var dump isPalindrome  madam       bool true  var dump isPalindrome  dell       bool false  var dump isPalindrome  1221       bool true

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Checking palindrome for first half of the string with the rest  this case assumes removal of any white spaces   public int isPalindrome String a              Remove all spaces and non alpha characters         String ab   a replaceAll    A-Za-z0-9        toLowerCase              System out println ab            for  int i 0  i lt ab length   2  i                  if ab charAt i     ab charAt  ab length  -1 -i                     return 0                                     return 1

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here  checking for the largest palindrome in a string  always starting from 1st char   public static String largestPalindromeInString String in        int right   in length   - 1      int left   0      char   word   in toCharArray        while  right  gt  left  amp  amp  word right     word left             right--            int lenght   right   1      while  right  gt  left  amp  amp  word right     word left              left            right--             if  0  gt   right - left            return new String Arrays copyOf word  lenght           else           return largestPalindromeInString                  new String Arrays copyOf word  in length   - 1

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Alternatively  recursion   For anybody who is looking for a shorter recursive solution  to check if a given string satisfies as a palindrome   private boolean isPalindrome String s        int length   s length         if  length  lt  2     If the string only has 1 char or is empty         return true      else              Check opposite ends of the string for equality         if  s charAt 0     s charAt length - 1               return false             Function call for string with the two ends snipped off         else             return isPalindrome s substring 1  length - 1                OR even shorter  if you d like   private boolean isPalindrome String s        int length   s length        if  length  lt  2  return true      return s charAt 0     s charAt length - 1    false               isPalindrome s substring 1  length - 1

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Here you can check palindrome a number of String dynamically  import java util Scanner   public class Checkpalindrome    public static void main String args        String original  reverse         Scanner in   new Scanner System in     System out println  Enter How Many number of Input you want         int numOfInt   in nextInt      original   in nextLine    do     if  numOfInt    0        System out println  Your Input Conplete            else       System out println  Enter a string to check palindrome        original   in nextLine         StringBuffer buffer   new StringBuffer original       reverse   buffer reverse   toString       if  original equalsIgnoreCase reverse         System out println  The entered string is Palindrome   reverse           else       System out println  The entered string is not Palindrome   reverse               numOfInt--        while  numOfInt  gt   0

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public class palindrome        public static void main String   args             Scanner scanner new Scanner System in           System out println  quot Enter the line you want to check palindrome  quot            String s  scanner nextLine             StringTokenizer separate   new StringTokenizer s   quot   quot            System out println  quot  nPalindrome Words are   quot            while separate hasMoreTokens                  String word   separate nextToken                String reversedWord   new StringBuilder word  reverse   toString   toLowerCase                if   word toLowerCase   equals reversedWord                     System out println word

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private static boolean isPalindrome String word             int z   word length            boolean isPalindrome   false           for  int i   0  i  lt   word length     2  i                  if  word charAt i     word charAt --z                     isPalindrome   true                                   return isPalindrome

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And here a complete Java 8 streaming solution  An IntStream provides all indexes til strings half length and then a comparision from the start and from the end is done   public static void main String   args        for  String testStr   Arrays asList  testset    none    andna    haah    habh    haaah              System out println  testing     testStr     is palindrome     isPalindrome testStr             public static boolean isPalindrome String str        return IntStream range 0  str length     2               noneMatch i - gt  str charAt i     str charAt str length   - i - 1        Output is   testing testset is palindrome true testing none is palindrome false testing andna is palindrome true testing haah is palindrome true testing habh is palindrome false testing haaah is palindrome true

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Why not just     boolean isPalindrom String s            char   myChars   s toCharArray            for  int i   0  i  lt  myChars length 2  i                  if  myChars i     myChars myChars length - 1 - i                     return false                                  return true

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package basicprogm   public class pallindrome      public static void main String   args           TODO Auto-generated method stub      String s   madam          to store the values that we got in loop     String t         for int i s length  -1 i gt  0 i--         t t s charAt i             System out println  reversed word is    t        if  t matches s          System out println  pallindrome              else        System out println  not pallindrome

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Try this out    import java util        public class str            public static void main String args                        Scanner in new Scanner System in             System out println  ENTER YOUR STRING                String a in nextLine              System out println  GIVEN STRING IS    a             StringBuffer str new StringBuffer a             StringBuffer str2 new StringBuffer str reverse               String s2 new String str2             System out println  THE REVERSED STRING IS    str2               if a equals s2                       System out println  ITS A PALINDROME                else                 System out println  ITS NOT A PALINDROME

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import java io    import java util     public class Solution        public static void main String   args             Scanner sc new Scanner System in           String A sc next            char   array   A toCharArray            String str               for int i A length  -1 i gt  0 i--               str   str   array i                     if A equalsIgnoreCase str  System out println  Yes            else System out println  No

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This implementation works for numbers and strings    Since we are not writing anything  so there is no need to convert the string into the character array       public static boolean isPalindrome Object obj        String s   String valueOf obj        for int left 0  right s length  -1  left  lt  right  left   right--                if s charAt left       s charAt right--               return false            return true

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import java util Scanner    public class Palindrom        public static void main String   args                Scanner in   new Scanner System in           String str  in nextLine            int x  str length             if x 2  0                        for int i 0 i lt x 2 i                                   if str charAt i   str charAt x-1-i                                         continue                                    else                                        System out println  String is not a palindrom                        break                                                    else                       for int i 0 i lt  x 2 i                                  if str charAt i   str charAt x-1-i                                         continue                                    else                                        System out println  String is not a palindrom                        break

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public class palindrome   public static void main String   args        StringBuffer strBuf1   new StringBuffer  malayalam        StringBuffer strBuf2   new StringBuffer  malayalam        strBuf2 reverse          System out println strBuf2       System out println  strBuf1 toString    equals strBuf2 toString          if   strBuf1 toString    equals strBuf2 toString             System out println  palindrome        else         System out println  not a palindrome

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import java util Collections  import java util HashSet  import java util Scanner  import java util Set   public class GetAllPalindromes        static Scanner in       public static void main String   args                 in   new Scanner System in           System out println  Enter a string  n            String abc   in nextLine            Set a   printAllPalindromes abc           System out println  set is       a              public static Set lt CharSequence gt  printAllPalindromes String input                 if  input length    lt   2                return Collections emptySet                       Set lt CharSequence gt  out   new HashSet lt CharSequence gt             int length   input length             for  int i   1  i  lt  length - 1  i                           for  int j   i - 1  k   i   1  j  gt   0  amp  amp  k  lt  length  j--  k                                   if  input charAt j     input charAt k                         out add input subSequence j  k   1                      else                       break                                                    return out             Get All Palindrome in s given string     Output  D  Java java GetAllPalindromes Enter a string  Hello user nitin is my best friend wow    Answer is set is    nitin   nitin    wow   wow  iti   D  Java

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public boolean isPalindrome String input        char   inputChars   input toCharArray        int inputLength   inputChars length      int inputMid   inputLength   2       for  int i   0  i  lt   inputMid  i              if  inputChars i     inputChars inputLength - i - 1                  return false                       return true      The method determines whether a string input is a palindrome  In this method the loop iterates for half of the input length resulting in less performance concern and more concise application

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public class Palindromes       public static void main String   args             String word    reliefpfpfeiller            char   warray   word toCharArray              System out println isPalindrome warray                      public static boolean isPalindrome char   word           if word length 2    0               for int i   0  i  lt  word length 2-1  i                     if word i     word word length-i-1                        return false                                           else              for int i   0  i  lt   word length-1  2-1  i                     if word i     word word length-i-1                        return false                                                    return true

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Why not just   public static boolean istPalindrom char   word       int i1   0      int i2   word length - 1      while  i2  gt  i1            if  word i1     word i2                 return false                      i1          --i2            return true      Example   Input is  andna   i1 will be 0 and i2 will be 4     First loop iteration we will compare word 0  and word 4   They re equal  so we increment i1  it s now 1  and decrement i2  it s now 3   So we then compare the n s  They re equal  so we increment i1  it s now 2  and decrement i2  it s 2   Now i1 and i2 are equal  they re both 2   so the condition for the while loop is no longer true so the loop terminates and we return true

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Code Snippet   import java util Scanner    class main        public static void main String   args               Scanner sc   new Scanner System in          String str   sc next           String reverse   new StringBuffer str  reverse   toString             if str equals reverse               System out println  Pallindrome            else             System out println  Not Pallindrome

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Amazing how many different solutions to such a simple problem exist  Here s another one         private static boolean palindrome String s       String revS           String checkS   s toLowerCase        String   checkSArr   checkS split           for String e   checkSArr           revS   e   revS             return  checkS equals revS     true   false

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Using Stream API    private static boolean isPalindrome char   warray        return IntStream range 0  warray length - 1               takeWhile i - gt  i  lt  warray length   2               noneMatch i - gt  warray i     warray warray length - 1 - i

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Go  Java   public boolean isPalindrome  String word        String myWord   word replaceAll    s            String reverse   new StringBuffer myWord  reverse   toString        return reverse equalsIgnoreCase myWord      isPalindrome  Never Odd or Even       True isPalindrome  Never Odd or Even1       False

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Another way is using char Array  public class Palindrome    public static void main String   args        String str    madam       if isPalindrome str             System out println  Palindrome          else           System out println  Not a Palindrome             private static boolean isPalindrome String str           Convert String to char array     char   charArray   str toCharArray          for int i 0  i  lt  str length    i              if charArray i     charArray  str length  -1  - i                 return false                      return true

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I was looking for a solution that not only worked for palindromes like       Kayak   Madam       but as well for       A man  a plan  a canal  Panama    Was it a car or a cat I saw    No  x  in Nixon    Iterative  This has be proven as a good solution   private boolean isPalindromeIterative final String string                final char   characters               string replaceAll     W        toLowerCase   toCharArray             int iteratorLeft   0          int iteratorEnd   characters length - 1           while  iteratorEnd  gt  iteratorLeft                        if  characters iteratorLeft       characters iteratorEnd--                                 return false                                   return true          Recursive  I think this solution shouldn t be much worse than the iterative one  Is a little bit crapy we need to extract the cleaning step out of the method to avoid unnecesary procesing   private boolean isPalindromeRecursive final String string                        final String cleanString   string replaceAll     W        toLowerCase                return isPalindromeRecursiveRecursion cleanString              private boolean isPalindromeRecursiveRecursion final String cleanString                        final int cleanStringLength   cleanString length                 return cleanStringLength  lt   1    cleanString charAt 0                            cleanString charAt cleanStringLength - 1   amp  amp                         isPalindromeRecursiveRecursion                               cleanString substring 1  cleanStringLength - 1                Reversing  This has been proved as a expensive solution   private boolean isPalindromeReversing final String string                final String cleanString   string replaceAll     W        toLowerCase            return cleanString equals new StringBuilder cleanString  reverse   toString             All the credits to the guys answering in this post and bringing light to the topic

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Here my analysis of the  Greg answer  componentsprogramming com palindromes     Sidenote  But  for me it is important to do it in a Generic way  The requirements are that the sequence is bidirectionally iterable and the elements of the sequence are comparables using equality  I don t know how to do it in Java  but  here is a C   version  I don t know a better way to do it for bidirectional sequences   template  lt BidirectionalIterator I gt       requires  EqualityComparable lt  ValueType lt I gt   gt     bool palindrome  I first  I last           I m   middle first  last        auto rfirst   boost  make reverse iterator last        return std  equal first  m  rfirst         Complexity  linear-time      If I is RandomAccessIterator   floor n 2  comparissons and floor n 2  2 iterations If I is BidirectionalIterator                floor n 2  comparissons and floor n 2  2 iterations              plus  3 2  n iterations to find the middle   middle function    storage  O 1  No dymamic allocated memory

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A concise version  that doesn t involve  inefficiently  initializing a bunch of objects   boolean isPalindrome String str            int n   str length        for  int i   0  i  lt  n 2  i             if  str charAt i     str charAt n-i-1   return false      return true

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I m new to java and I m taking up your question as a challenge to improve my knowledge   import java util ArrayList  import java util List   public class PalindromeRecursiveBoolean        public static boolean isPalindrome String str             str   str toUpperCase            char   strChars   str toCharArray             List lt Character gt  word   new ArrayList lt  gt             for  char c   strChars                word add c                      while  true                if   word size      1      word size      0                     return true                            if  word get 0     word get word size   - 1                     word remove 0                   word remove word size   - 1                 else                   return false                                       If the string is made of no letters or just one letter  it is a palindrome  Otherwise  compare the first and last letters of the string   If the first and last letters differ  then the string is not a palindrome Otherwise  the first and last letters are the same  Strip them from the string  and determine whether the string that remains is a palindrome  Take the answer for this smaller string and use it as the answer for the original string then repeat from 1

[java] Check string for palindrome

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