How to check if the given string is palindrome

Question

Definition   A palindrome is a word  phrase  number or other sequence of units that has the property of reading the same in either direction  How to check if the given string is a palindrome   This was one of the FAIQ  Frequently Asked Interview Question  a while ago but that mostly using C   Looking for solutions in any and all languages possible

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Const-correct C C   pointer solution  Minimal operations in loop   int IsPalindrome  const char  str        const unsigned len   strlen str       const char  end    amp str len-1       while  str  lt  end          if   str       end--              return 0      return 1

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C in the house   not sure if you didn t want a C example here   bool IsPalindrome char  s        int  i d      int  length   strlen s       char cf  cb       for i 0  d length-1   i  lt  length  amp  amp  d  gt   0   i     d--                while cf  toupper s i     cf  lt   A     cf  gt  Z    amp  amp  i  lt  length-1 i            while cb  toupper s d     cb  lt   A     cb  gt  Z    amp  amp  d  gt  0        d--          if cf    cb  amp  amp  cf  gt    A   amp  amp  cf  lt    Z   amp  amp  cb  gt    A   amp  amp  cb  lt   Z               return false            return true      That will return true for  racecar    Racecar    race car    racecar    and  RaCe cAr    It would be easy to modify to include symbols or spaces as well  but I figure it s more useful to only count letters and ignore case    This works for all palindromes I ve found in the answers here  and I ve been unable to trick it into false negatives positives   Also  if you don t like bool in a  C  program  it could obviously return int  with return 1 and return 0 for true and false respectively

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How about this PHP example    function noitcnuf   returnstrrevverrtsnruter   functionnoitcnuf         returnstrrev     returnstrrevverrtsnruter        verrtsnruter    functionnoitcnuf      return  strrev   verrtsnruter      functionnoitcnuf

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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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Java solution   public class QuickTest    public static void main String   args        check  AmanaplanacanalPanama  toLowerCase         check  Hello World  toLowerCase        public static void check String aString        System out print aString              char   chars   aString toCharArray        for  int i   0  j    chars length - 1   i  lt   chars length   2   i    j--            if  chars i     chars j                 System out println  Not a palindrome                 return                      System out println  Found a palindrome

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Java with stacks   public class StackPalindrome       public boolean isPalindrome String s  throws OverFlowException EmptyStackException          boolean isPal false          String pal             char letter          if  s                   return true          else                 s s toLowerCase                for int i 0 i lt s length   i                  letter s charAt i                char   alphabet   a   b   c   d   e   f   g   h   i   j   k   l   m   n   o   p   q   r   s   t   u   v   w   x   y   z                for int j 0  j lt alphabet length j                       removing punctuations                   if   String valueOf letter   equals String valueOf alphabet j                          pal   letter                                                      int len pal length            char   palArray new char len           for int r 0 r lt len r                 palArray r  pal charAt r                     ArrayStack palStack new ArrayStack len           for int k 0 k lt palArray length k                 palStack push palArray k                      for  int i 0 i lt len i                  if   palStack topAndpop    equals palArray i                    isPal true              else                  isPal false                return isPal          public static void main  String args    throws EmptyStackException OverFlowException       StackPalindrome s new StackPalindrome        System out println s isPalindrome     Ma     Jerome raps pot top     Spare more jam

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Here s my solution in c   static bool isPalindrome string s        string allowedChars    abcdefghijklmnopqrstuvwxyz            1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ       string compareString   String Empty      string rev   string Empty       for  int i   0  i  lt   s Length - 1  i                  char c   s i            if  allowedChars IndexOf c   gt  -1                        compareString    c                        for  int i   compareString Length - 1  i  gt   0  i--                char c   compareString i           rev    c             return rev Equals compareString           StringComparison CurrentCultureIgnoreCase

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Language agnostic meta-code then     rev   StringReverse originalString  return   rev    originalString

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boolean isPalindrome String str1        first strip out punctuation and spaces   String stripped   str1 replaceAll    a-zA-Z0-9           return stripped equalsIgnoreCase  new StringBuilder stripped   reverse   toString         Java version

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If we re looking for numbers and simple words  many correct answers have been given   However  if we re looking for what we generally see as palindromes in written language  e g    A dog  a panic  in a pagoda     the correct answer would be to iterate starting from both ends of the sentence  skipping non-alphanumeric characters individually  and returning false if any mismatches are found   i   0  j   length-1   while  true       while  i  lt  j  amp  amp   is alphanumeric  str i      i      while  i  lt  j  amp  amp   is alphanumeric  str j      j--     if  i  gt   j   return true     if  tolower string i      tolower string j     return false    i    j--        Of course  stripping out non-valid characters  reversing the resulting string and comparing it to the original one also works  It comes down to what type of language you re working on

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Regular Approach   flag   True    Assume palindrome is true for i from 0 to n 2     compare element i  and element n-i-1     0 to n-1    if not equal set flag   False    break   return flag   There is a better machine optimized method which uses XORs but has the same complexity  XOR approach   n   length of string mid element   element n 2  1  for i from 0 to n   t xor   element i  xor element i 1    if n is odd compare mid element and t xor else check t xor is zero

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Delphi  function IsPalindrome const s  string   boolean  var   i  j  integer  begin   Result    false    j    Length s     for i    1 to Length s  div 2 do begin     if s i   lt  gt  s j  then       Exit      Dec j     end    Result    true  end

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Windows XP  might also work on 2000  or later BATCH script    echo off  call  is palindrome  1 if  ERRORLEVEL     0       echo  1 is a palindrome   else       echo  1 is NOT a palindrome   exit  B 0   is palindrome     set word   1     set reverse      call  reverse chars   word       set return 1     if    word         reverse             set return 0       exit  B  return    reverse chars     set chars   1     set reverse  chars  0 1  reverse      set chars  chars  1      if    chars                    exit  B 0       else           call  reverse chars   chars         exit  B 0

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Ruby   class String     def is palindrome          letters only   gsub   W      downcase         letters only    letters only reverse     end end  puts  abc  is palindrome      gt  false puts  aba  is palindrome      gt  true puts  Madam  I m Adam   is palindrome      gt  true

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C   LINQ  var str    a b a   var test   Enumerable SequenceEqual str ToCharArray                str ToCharArray   Reverse

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Easy mode in C   only using Base Class Libraries  Edit  just saw someone did Array Reverse also    public bool IsPalindrome string s                                if  String IsNullOrEmpty s                                         return false                                     else                                       char   t   s ToCharArray                        Array Reverse t                       string u   new string t                       if  s ToLower      u ToLower                                                  return true                                                           return false

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Python   if s    s   -1   return True   Java   if  s Equals s Reverse       return true      PHP   if  s    strrev s   return true    Perl   if  s    reverse s     return true      Erlang   string equal S  lists reverse S

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Single program for Both String or Integer to check palindrome    In java with out using string functions like reverse and equals method also and display matching characters also  package com practice   import java util Scanner   public class Pallindrome            public static void main String args              Scanner sc new Scanner System in           int i 0 j 0 k count 0          String input temp          System out println  Enter the String or Integer            input sc nextLine            temp input          k temp length  -1          for i 0 i lt  input length  -1 i                  if input charAt j   temp charAt k                     count                                For matching characters             j                k--                            System out println  Matching Characters     count            if count  input length                  System out println  It s a pallindrome                      else               System out println  It s not a pallindrome

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public bool IsPalindrome string s                string formattedString   s Replace      string Empty  ToLower            for  int i   0  i  lt  formattedString Length   2  i                          if  formattedString i     formattedString formattedString Length - 1 - i                   return false                    return true          This method will work for sting like  Was it a rat I saw   But I feel we need to eliminate special character through Regex

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OCaml   let rec palindrome s     s    tailrev s    source

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Prolog   palindrome B  R   - palindrome B  R        palindrome     R  R   palindrome  X B    X R   T   - palindrome B  R   X T

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This Java code should work inside a boolean method   Note  You only need to check the first half of the characters with the back half  otherwise you are overlapping and doubling the amount of checks that need to be made   private static boolean doPal String test        for int i   0  i  lt  test length     2  i              if test charAt i     test charAt test length   - 1 - i                 return false                      return true

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Here s another for C  that I used when doing a sample server control  It can be found in the book ASP NET 3 5 Step by Step  MS Press   It s two methods  one to strip non-alphanumerics  and another to check for a palindrome   protected string StripNonAlphanumerics string str        string strStripped    String str Clone        if  str    null                char   rgc   strStripped ToCharArray            int i   0          foreach  char c in rgc                        if  char IsLetterOrDigit c                                 i                              else                               strStripped   strStripped Remove i  1                                     return strStripped    protected bool CheckForPalindrome         if  this Text    null                String strControlText   this Text          String strTextToUpper   null          strTextToUpper   Text ToUpper            strControlText                       this StripNonAlphanumerics strTextToUpper           char   rgcReverse   strControlText ToCharArray            Array Reverse rgcReverse           String strReverse   new string rgcReverse           if  strControlText    strReverse                        return true                    else                       return false                      else               return false

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Unoptimized Python    gt  gt  gt  def is palindrome s           return s    s   -1

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I m seeing a lot of incorrect answers here  Any correct solution needs to ignore whitespace and punctuation  and any non-alphabetic characters actually  and needs to be case insensitive   A few good example test cases are    A man  a plan  a canal  Panama     A Toyota s a Toyota     A       As well as some non-palindromes   Example solution in C   note  empty and null strings are considered palindromes in this design  if this is not desired it s easy to change    public static bool IsPalindrome string palindromeCandidate        if  string IsNullOrEmpty palindromeCandidate                 return true            Regex nonAlphaChars   new Regex    a-z0-9         string alphaOnlyCandidate   nonAlphaChars Replace palindromeCandidate ToLower             if  string IsNullOrEmpty alphaOnlyCandidate                 return true            int leftIndex   0      int rightIndex   alphaOnlyCandidate Length - 1      while  rightIndex  gt  leftIndex                if  alphaOnlyCandidate leftIndex     alphaOnlyCandidate rightIndex                         return false                    leftIndex            rightIndex--            return true

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I had to do this for a programming challenge  here s a snippet of my Haskell   isPalindrome    String - gt  Bool isPalindrome n    n    reverse n

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A more Ruby-style rewrite of Hal s Ruby version   class String   def palindrome       test   gsub    A-Za-z        downcase     test reverse   end end   Now you can call palindrome  on any string

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Erlang is awesome  palindrome L  - gt  palindrome L       palindrome       - gt  false  palindrome            - gt  true  palindrome    L  L  - gt  true  palindrome L L  - gt  true  palindrome  H T   Acc  - gt  palindrome T   H Acc

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I haven t seen any recursion yet  so here goes     import re  r   re compile    0-9a-zA-Z     def is pal s        def inner pal s           if len s     0              return True         elif s 0     s -1               return inner pal s 1 -1           else              return False      r   re compile    0-9a-zA-Z        return inner pal r sub     s  lower

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JavaScript Version  function isPalindrome str       str   str replace    a-zA-Z  g        return str split     reverse   join     toUpperCase       str toUpperCase

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public static boolean isPalindrome String str        return str equals new StringBuilder str  reverse   toString         for versions of Java earlier than 1 5   public static boolean isPalindrome String str        return str equals new StringBuffer   append str  reverse   toString         or  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

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In Ruby  converting to lowercase and stripping everything not alphabetic   def isPalindrome  string         test   string downcase gsub     a-z              test reverse end   But that feels like cheating  right  No pointers or anything  So here s a C version too  but without the lowercase and character stripping goodness    include  lt stdio h gt  int isPalindrome  char   string         char   i   string      char   p   string      while      i    while   i  gt  p  amp  amp   p       --i        return i  lt   p  amp  amp   i       --p    int main  int argc  char   argv         if   argc    2                 fprintf  stderr   Usage   s  lt word gt  n   argv 0             return -1            fprintf  stdout    s n   isPalindrome  argv 1       yes     no         return 0      Well  that was fun - do I get the job

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Scala  def pal s String    Symbol s  equals Symbol s reverse

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Many ways to do it  I guess the key is to do it in the most efficient way possible  without looping the string   I would do it as a char array which can be reversed easily  using C     string mystring    abracadabra    char   str   mystring ToCharArray    Array Reverse str   string revstring   new string str    if  mystring equals revstring         Console WriteLine  String is a Palindrome

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boolean IsPalindrome string s    return s   s Reverse

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No solution using JavaScript yet   function palindrome s      var l   0  r   s length - 1    while  l  lt  r  if  s charAt left        s charAt r--   return false    return true

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Note that in the above C   solutions  there was some problems  One solution was inefficient because it passed an std  string by copy  and because it iterated over all the chars  instead of comparing only half the chars  Then  even when discovering the string was not a palindrome  it continued the loop  waiting its end before reporting  quot false quot   The other was better  with a very small function  whose problem was that it was not able to test anything else than std  string  In C    it is easy to extend an algorithm to a whole bunch of similar objects  By templating its std  string into  quot T quot   it would have worked on both std  string  std  wstring  std  vector and std  deque  But without major modification because of the use of the operator  lt   the std  list was out of its scope  My own solutions try to show that a C   solution won t stop at working on the exact current type  but will strive to work an anything that behaves the same way  no matter the type  For example  I could apply my palindrome tests on std  string  on vector of int or on list of  quot Anything quot  as long as Anything was comparable through its operator    build in types  as well as classes   Note that the template can even be extended with an optional type that can be used to compare the data  For example  if you want to compare in a case insensitive way  or even compare similar characters  like                and e   Like king Leonidas would have said   quot Templates   This is C       quot  So  in C    there are at least 3 major ways to do it  each one leading to the other  Solution A  In a c-like way The problem is that until C  0X  we can t consider the std  string array of chars as contiguous  so we must  quot cheat quot  and retrieve the c str   property  As we are using it in a read-only fashion  it should be ok     bool isPalindromeA const std  string  amp  p strText       if p strText length    lt  2  return true      const char   pStart   p strText c str                     const char   pEnd   pStart   p strText length   - 1        for   pStart  lt  pEnd    pStart  --pEnd             if  pStart     pEnd                   return false                    return true       Solution B  A more  quot C   quot  version Now  we ll try to apply the same solution  but to any C   container with random access to its items through operator     For example  any std  basic string  std  vector  std  deque  etc  Operator    is constant access for those containers  so we won t lose undue speed   template  lt typename T gt  bool isPalindromeB const T  amp  p aText       if p aText empty    return true      typename T  size type iStart   0      typename T  size type iEnd   p aText size   - 1       for   iStart  lt  iEnd    iStart  --iEnd             if p aText iStart     p aText iEnd                    return false                    return true       Solution C  Template powah   It will work with almost any unordered STL-like container with bidirectional iterators For example  any std  basic string  std  vector  std  deque  std  list  etc  So  this function can be applied on all STL-like containers with the following conditions  1 - T is a container with bidirectional iterator 2 - T s iterator points to a comparable type  through operator     template  lt typename T gt  bool isPalindromeC const T  amp  p aText       if p aText empty    return true      typename T  const iterator pStart   p aText begin        typename T  const iterator pEnd   p aText end        --pEnd       while true             if  pStart     pEnd                   return false                  if  pStart    pEnd        pStart    pEnd                    return true                  --pEnd

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A simple Java solution   public boolean isPalindrome String testString        StringBuffer sb   new StringBuffer testString       String reverseString   sb reverse   toString         if testString equalsIgnoreCase reverseString             return true      else           return false

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C 3 - This returns false as soon as a char counted from the beginning fails to match its equivalent at the end   static bool IsPalindrome this string input        char   letters   input ToUpper   ToCharArray         int i   0      while  i  lt  letters Length   2           if  letters i     letters letters Length -   i                return false       return true

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There isn t a single solution on here which takes into account that a palindrome can also be based on word units  not just character units   Which means that none of the given solutions return true for palindromes like  Girl  bathing on Bikini  eyeing boy  sees boy eyeing bikini on bathing girl    Here s a hacked together version in C   I m sure it doesn t need the regexes  but it does work just as well with the above bikini palindrome as it does with  A man  a plan  a canal-Panama         static bool IsPalindrome string text                bool isPalindrome   IsCharacterPalindrome text           if   isPalindrome                        isPalindrome   IsPhrasePalindrome text                     return isPalindrome             static bool IsCharacterPalindrome string text                String clean   Regex Replace text ToLower       A-z0-9    String Empty  RegexOptions Compiled           bool isPalindrome   false          if   String IsNullOrEmpty clean   amp  amp  clean Length  gt  1                        isPalindrome   true              for  int i   0  count   clean Length   2   1  i  lt  count  i                                  if  clean i     clean clean Length - 1 - i                                         isPalindrome   false  break                                                    return isPalindrome             static bool IsPhrasePalindrome string text                bool isPalindrome   false          String clean   Regex Replace text ToLower        A-z0-9 s         RegexOptions Compiled  Trim            String   words   Regex Split clean     s             if  words Length  gt  1                        isPalindrome   true              for  int i   0  count   words Length   2   1  i  lt  count  i                                  if  words i     words words Length - 1 - i                                         isPalindrome   false  break                                                    return isPalindrome

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Here s a python way  Note  this isn t really that  pythonic  but it demonstrates the algorithm   def IsPalindromeString n       myLen   len n      i   0     while i  lt   myLen 2          if n i     n myLen-1-i               return False         i    1     return True

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Three versions in Smalltalk  from dumbest to correct     In Smalltalk    is the comparison operator   isPalindrome  aString      Dumbest         aString reverse   aString     The message  translateToLowercase returns the string as lowercase   isPalindrome  aString      Case insensitive       lowercase      lowercase    aString translateToLowercase        lowercase reverse   lowercase     And in Smalltalk  strings are part of the Collection framework  you can use the message  select thenCollect   so here s the last version   isPalindrome  aString      Case insensitive and keeping only alphabetic chars      blanks  amp  punctuation insensitive         lowercaseLetters      lowercaseLetters    aString         select    char   char isAlphabetic          thenCollect    char   char asLowercase          lowercaseLetters reverse   lowercaseLetters

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PHP sample    string    A man  a plan  a canal  Panama    function is palindrome  string         a   strtolower preg replace     A-Za-z0-9        string        return  a  strrev  a       Removes any non-alphanumeric characters  spaces  commas  exclamation points  etc   to allow for full sentences as above  as well as simple words

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Another C   one  Optimized for speed and size   bool is palindrome const std  string amp  candidate        for std  string  const iterator left   candidate begin    right   candidate end    left  lt  --right     left          if   left     right              return false      return true

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C  in-place algorithm  Any preprocessing  like case insensitivity or stripping of whitespace and punctuation should be done before passing to this function   boolean IsPalindrome string s        for  int i   0  i  lt  s Length   2  i                  if  s i     s s Length - 1 - i   return false            return true        Edit  removed unnecessary   1  in loop condition and spent the saved comparison on removing the redundant Length comparison  Thanks to the commenters

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My 2c   Avoids overhead of full string reversal everytime  taking advantage of shortcircuiting to return as soon as the nature of the string is determined   Yes  you should condition your string first  but IMO that s the job of another function   In C            lt summary gt          Tests if a string is a palindrome          lt  summary gt      public static bool IsPalindrome this String str                if  str Length    0  return false          int index   0          while  index  lt  str Length   2              if  str index     str str Length -   index   return false           return true

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EDIT  from the comments   bool palindrome std  string const amp  s        return std  equal s begin    s end    s rbegin             The c   way   My naive implementation using the elegant iterators  In reality  you would probably check and stop once your forward iterator has past the halfway mark to your string    include  lt string gt   include  lt iostream gt   using namespace std  bool palindrome string foo        string  iterator front      string  reverse iterator back      bool is palindrome   true      for front   foo begin    back   foo rbegin            is palindrome  amp  amp  front   foo end    amp  amp  back    foo rend              front    back                         if  front     back              is palindrome   false            return is palindrome    int main         string a    hi there   b    laval        cout  lt  lt   String a       lt  lt  a  lt  lt      is    lt  lt    palindrome a          not     lt  lt   a palindrome    lt  lt endl      cout  lt  lt   String b       lt  lt  b  lt  lt      is    lt  lt    palindrome b          not     lt  lt   a palindrome    lt  lt endl

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public static boolean isPalindrome  String str         int count   str length         int i  j   count - 1       for   i   0   i  lt  count   i               if   str charAt i     str charAt j    return false           if   i    j   return true           j--             return true

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Another one from Delphi  which I think is a little more rigorous than the other Delphi example submitted   This can easily turn into a golfing match  but I ve tried to make mine readable   Edit0  I was curious about the performance characteristics  so I did a little test   On my machine  I ran this function against a 60 character string 50 million times  and it took 5 seconds   function TForm1 IsPalindrome txt  string   boolean  var   i  halfway  len   integer  begin   Result    True    len    Length txt          special cases    an empty string is  never  a palindrome   a 1-character string is  always  a palindrome       case len of     0   Result    False      1   Result    True      else begin       halfway    Round  len 2  -  1 2       if odd  round down to get 1 2way pt          scan half of our string  make sure it is mirrored on the other half       for i    1 to halfway do begin         if txt i   lt  gt  txt len- i-1   then begin           Result    False            Break          end     if we found a non-mirrored character       end     for 1st half of string     end     else not a special case   end     case end    And here is the same thing  in C   except that I ve left it with multiple exit points  which I don t like   private bool IsPalindrome string txt      int len   txt Length          Special cases    An empty string is  never  a palindrome   A 1-character string is  always  a palindrome            switch  len        case 0  return false      case 1  return true         switch   int halfway    len   2        scan half of our string  make sure it is mirrored on the other half   for  int i   0  i  lt  halfway    i        if  txt Substring i 1     txt Substring len - i - 1 1           return false           if        for   return true

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If you can use Java APIs and additional storage   public static final boolean isPalindromeWithAdditionalStorage String string        String reversed   new StringBuilder string  reverse   toString        return string equals reversed       In can need an in-place method for Java   public static final boolean isPalindromeInPlace String string        char   array   string toCharArray        int length   array length-1      int half   Math round array length 2       char a b      for  int i length  i gt  half  i--            a   array length-i           b   array i           if  a    b  return false            return true

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The interviewer will be looking for some logic on how you will be approaching this problem  Please consider the following java code    always check if the input string is null check your base cases  format your string accordingly  remove anything that is not a character digit Most likely they do not want to see if you know the reverse function  and a comparison  but rather if you are able to answer the question using a loop and indexing  shortcut return as soon as you know your answer and do not waste resources for nothing   public static boolean isPalindrome String s     if  s    null    s length      0    s length      1      return false   String ss   s toLowerCase   replaceAll     a-z           for  int i   0  i  lt  ss length   2  i         if  ss charAt i     ss charAt ss length   - 1 - i           return false  return true

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set l   index of left most character in word set r   index of right most character in word  loop while l  lt  r  begin   if letter at l does not equal letter at r     word is not palindrome   else      increase l and decrease r end word is palindrome

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An obfuscated C version   int IsPalindrome  char  s      char a  b c 0    for a b s a lt  b c  c c  1 c   a amp  32 -65 gt 25u    a 1 2 c  2   --b amp  32 -65 lt 26u 3 2 c  3   b-65 amp  32 -  a-65 amp  32    b s 0 a  4    a 1 0    b 0 1      return s  0

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Lisp    defun palindrome x   string  x  reverse x

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Perl   sub is palindrome       my  s   lc shift    normalize case      s    s  W  g       strip non-word characters     return  s eq reverse  s

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Here s a Python version that deals with different cases  punctuation and whitespace   import string  def is palindrome palindrome       letters   palindrome translate string maketrans                           string whitespace   string punctuation  lower       return letters    letters   -1    Edit  Shamelessly stole from Blair Conrad s neater answer to remove the slightly clumsy list processing from my previous version

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The solutions which strip out any chars that don t fall between A-Z or a-z are very English centric  Letters with diacritics such as    or    would be stripped    According to Wikipedia       The treatment of diacritics varies  In languages such as Czech and Spanish  letters with diacritics or accents  except tildes  are not given a separate place in the alphabet  and thus preserve the palindrome whether or not the repeated letter has an ornamentation  However  in Swedish and other Nordic languages  A and A with a ring      are distinct letters and must be mirrored exactly to be considered a true palindrome    So to cover many other languages it would be better to use collation to convert diacritical marks to their equivalent non diacritic or leave alone as appropriate and then strip whitespace and punctuation only before comparing

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Efficient C   version   template lt  typename Iterator  gt  bool is palindrome  Iterator first  Iterator last  std  locale const amp  loc   std  locale             if   first    last           return true       for  --last  first  lt  last    first  --last                 while    std  isalnum   first  loc    amp  amp  first  lt  last                 first          while    std  isalnum   last  loc    amp  amp  first  lt  last               --last          if   std  tolower   first  loc      std  tolower   last  loc                 return false            return true

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Perl   sub is palindrome         s   lc shift     ignore case    s    s  W   g    consider only letters  digits  and        s eq reverse  s      It ignores case and strips non-alphanumeric characters  it locale- and unicode- neutral

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Here s my solution  without using a strrev  Written in C   but it will work in any language that has a string length function   private static bool Pal string s        for  int i   0  i  lt  s Length  i              if  s i     s s Length - 1 - i                 return false                      return true

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A case-insensitive  const-friendly version in plain C that ignores non-alphanumeric characters  e g  whitespace   punctuation   It therefore will actually pass on classics like  A man  a plan  a canal  Panama    int ispalin const char  b        const char  e   b      while     e       while  --e  gt   b                if  isalnum  e                         while   b  amp  amp   isalnum  b     b              if  toupper  b       toupper  e   return 0                      return 1

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C    std  string a    god   std  string b    lol    std  cout  lt  lt   std  string a rbegin    a rend       a   lt  lt                  lt  lt   std  string b rbegin    b rend       b     Bash  function ispalin         echo -n  1   tac -rs          1       echo    ispalin god  amp  amp  echo yes    echo no     ispalin lol  amp  amp  echo yes    echo no     Gnu Awk     obvious solution    function ispalin cand  i         for i 0  i lt length cand  2  i             if substr cand  length cand -i  1     substr cand  i 1  1                return 0       return 1         not so obvious solution  cough cough           orig    0      while  0             stuff   stuff gensub               1   1             0   gensub               1   1              print  stuff    orig        Haskell  Some brain dead way doing it in Haskell  ispalin     Char  - gt  Bool ispalin a   a     let xi  y my     xi my      y   xi         in  x - gt  xi x  a   Plain English   Just reverse the string and if it is the same as before  it s a palindrome

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C  Version   Assumes MyString to be a char    Method return after verification of the string  it ignores space and  lt     but this can be extended to ignore more  probably impleemnt a regex match of ignore list   public bool IsPalindrome               if  MyString Length    0                  return false               int len   MyString Length - 1               int first   0              int second   0               for  int i   0  j   len  i  lt   len   2  i    j--                                while  i lt j  amp  amp  MyString i            MyString i                              i                     while j gt i  amp  amp  MyString j            MyString j                              j--                   if   i    len   2   amp  amp   i    j                       return true                   first   MyString i   gt   97  amp  amp  MyString i   lt   122   MyString i  - 32   MyString i                   second   MyString j   gt   97  amp  amp  MyString j   lt   122   MyString j  - 32   MyString j                    if  first    second                      return false                             return true        Quick test cases  negative 1  ABCDA  2  AB CBAG 3  A  BDA 4  NULL EMPTY  positive 1  ABCBA 2  A  man a plan a canal  Panama 3  ABC BA 4  M 5  ACCB  let me know any thoghts errors

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Using Java  using Apache Commons String Utils   public boolean isPalindrome String phrase      phrase   phrase toLowerCase   replaceAll    a-z           return StringUtils reverse phrase  equals phrase

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Here are two more Perl versions  neither of which uses reverse   Both use the basic algorithm of comparing the first character of the string to the last  then discarding them and repeating the test  but they use different methods of getting at the individual characters  the first peels them off one at a time with a regex  the second splits the string into an array of characters       usr bin perl  my  strings     A man  a plan  a canal  Panama     A Toyota s a Toyota                     A        As well as some non-palindromes      for my  string   strings      print is palindrome  string        string  is a palindrome  1  n                                       string  is not a palindrome  1  n     print is palindrome2  string       string  is a palindrome  2  n                                       string  is not a palindrome  2  n       sub is palindrome     my  str   lc shift     str    tr a-z  cd     while   str    s               2         return unless  1 eq  3         return 1      sub is palindrome2     my  str   lc shift     str    tr a-z  cd    my  chars   split      str     while   chars  amp  amp  shift  chars eq pop  chars         return scalar  chars  lt   1

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Simple JavaScript solution  Run code snippet for demo    x000D   x000D  function checkPalindrome line   x000D        reverse line line split     reverse   join      x000D        return line  reverse line  x000D      x000D  alert  checkPalindrome radar     checkPalindrome  radar     x000D   x000D   x000D

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This is all good  but is there a way to do better algorithmically  I was once asked in a interview to recognize a palindrome in linear time and constant space   I couldn t think of anything then and I still can t     If it helps  I asked the interviewer what the answer was  He said you can construct a pair of hash functions such that they hash a given string to the same value if and only if that string is a palindrome  I have no idea how you would actually make this pair of functions

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Using a good data structure usually helps impress the professor   Push half the chars onto a stack  Length   2   Pop and compare each char until the first unmatch  If the stack has zero elements  palindrome   in the case of a string with an odd Length  throw out the middle char

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c     bool is palindrome const string  amp s        return equal  s begin    s begin   s length   2  s rbegin

[string] How to check if the given string is palindrome?

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