Creating a recursive method for Palindrome

Question

I am trying to create a Palindrome program using recursion within Java but I am stuck  this is what I have so far    public static void main  String   args    System out println isPalindrome  noon      System out println isPalindrome  Madam I m Adam      System out println isPalindrome  A man  a plan  a canal  Panama      System out println isPalindrome  A Toyota      System out println isPalindrome  Not a Palindrome      System out println isPalindrome  asdfghfdsa        public static boolean isPalindrome String in    if in equals         in length      1   return true   in  in toUpperCase     if Character isLetter in charAt 0      public static boolean isPalindromeHelper String in    if in equals        in length    1     return true             Can anyone supply a solution to my problem

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I think  recursion isn t the best way to solve this problem  but one recursive way I see here is shown below   String str   prepareString originalString     make upper case  remove some characters  isPalindrome str    public boolean isPalindrome String str       return str length      1    isPalindrome str  0      private boolean isPalindrome String str  int i           if  i  gt  str length   2          return true          if   str charAt i  equals str charAt str length   - 1 - i            return false          return isPalindrome str  i 1

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Palindrome example   static boolean isPalindrome String sentence           If the length of the string is 0 or 1 no more string to check          return true  as the base case  Then compare to see if the first        and last letters are equal  by cutting off the first and last        letters each time the function is recursively called         int length   sentence length         if  length  gt   1          return true      else           char first   Character toLowerCase sentence charAt 0            char last   Character toLowerCase sentence charAt length-1             if  Character isLetter first   amp  amp  Character isLetter last                 if  first    last                    String shorter   sentence substring 1  length-1                   return isPalindrome shorter                 else                   return false                          else if   Character isLetter last                 String shorter   sentence substring 0  length-1               return isPalindrome shorter             else               String shorter   sentence substring 1               return isPalindrome shorter                       Called by    System out println r isPalindrome  Madam  I m Adam       Will print true if palindrome  will print false if not   If the length of the string is 0 or 1 no more string to check   return true  as the base case  This base case will be referred to by function call right before this  Then compare to see if the first and last letters are equal  by cutting off the first and last letters each time the function is recursively called

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Here is the code for palindrome check without creating many strings  public static boolean isPalindrome String str       return isPalindrome str 0 str length  -1      public static boolean isPalindrome String str  int start  int end       if start  gt   end          return true      else         return  str charAt start     str charAt end    amp  amp  isPalindrome str  start 1  end-1

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public static boolean isPalindrome String p                if p length      0    p length      1                 if length  0 OR 1 then it is             return true             if p substring 0 1  equalsIgnoreCase p substring p length  -1                 return isPalindrome p substring 1  p length  -1              return false          This solution is not case sensitive  Hence  for example  if you have the following word    adinida   then  you will get true if you do  Adninida  or  adninida  or  adinidA   which is what we want    I like  JigarJoshi answer  but the only problem with his approach is that it will give you false for words which contains caps

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Some of the codes are string heavy  Instead of creating substring which creates new object  we can just pass on indexes in recursive calls like below   private static boolean isPalindrome String str  int left  int right        if left  gt   right            return true            else           if str charAt left     str charAt right                  return isPalindrome str    left  --right                     else               return false                       public static void main String   args       String str    abcdcbb        System out println isPalindrome str  0  str length  -1

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Well    It s not clear why you ve got two methods with the same signature  What are they meant to accomplish  In the first method  why are you testing for testing for a single space or any single character  You might want to consider generalizing your termination condition to  if the length is less than two  Consider how you want to recurse  One option    Check that the first letter is equal to the last letter  If not  return false Now take a substring to effectively remove the first and last letters  and recurse  Is this meant to be an exercise in recursion  That s certainly one way of doing it  but it s far from the only way    I m not going to spell it out any more clearly than that for the moment  because I suspect this is homework - indeed some may consider the help above as too much  I m certainly slightly hesitant myself   If you have any problems with the above hints  update your question to show how far you ve got

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Here are three simple implementations  first the oneliner   public static boolean oneLinerPalin String str       return str equals new StringBuffer str  reverse   toString         This is ofcourse quite slow since it creates a stringbuffer and reverses it  and the whole string is always checked nomatter if it is a palindrome or not  so here is an implementation that only checks the required amount of chars and does it in place  so no extra stringBuffers   public static boolean isPalindrome String str        if str isEmpty    return true       int last   str length   - 1               for int i   0  i  lt   last   2 i            if str charAt i     str charAt last - i               return false       return true      And recursively   public static boolean recursivePalin String str       return check str  0  str length   - 1      private static boolean check  String str int start int stop       return stop - start  lt  2               str charAt start     str charAt stop   amp  amp             check str  start   1  stop - 1

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Function to check a String is palindrome or not         param s input String         return true if Palindrome             public boolean checkPalindrome String s             if  s length      1    s isEmpty                return true           boolean palindrome   checkPalindrome s substring 1  s length   - 1             return palindrome  amp  amp  s charAt 0     s charAt s length   - 1

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Simple Solution 2 Scenario -- Odd or Even length  String   Base condition amp  Algo recursive ch  i  j    i  j    even len if i lt  j recurve call  ch  i  1 j-1  else return ch i    ch j    Extra base condition for old length    public class HelloWorld      static boolean ispalindrome char ch    int i  int j      if  i    j  return true     if  i  lt  j       if  ch i     ch j       return false     else     return ispalindrome ch  i   1  j - 1         if  ch i     ch j      return false    else    return true       public static void main String   args      System out println ispalindrome  jatin  toCharArray    0  4      System out println ispalindrome  nitin  toCharArray    0  4      System out println ispalindrome  jatinn  toCharArray    0  5      System out println ispalindrome  nittin  toCharArray    0  5

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public class palin        static boolean isPalin String s  int i  int j                boolean b true          if s charAt i   s charAt j                         if i lt  j                  isPalin s  i 1   j-1                      else                       b false                    return b            public static void main                 String s1  madam           if isPalin s1  0  s1 length  -1   true              System out println s1   is palindrome            else             System out println s1   is not palindrome

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Here is my go at it   public class Test        public static boolean isPalindrome String s            return s length    lt   1                 s charAt 0     s charAt s length   - 1   amp  amp               isPalindrome s substring 1  s length   - 1                 public static boolean isPalindromeForgiving String s            return isPalindrome s toLowerCase   replaceAll     s  pP                      public static void main String   args                True  odd length          System out println isPalindrome  asdfghgfdsa                 True  even length          System out println isPalindrome  asdfggfdsa                 False         System out println isPalindrome  not palindrome                 True  but very forgiving            System out println isPalindromeForgiving  madam I m Adam

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for you to achieve that  you not only need to know how recursion works but you also need to understand the String method   here is a sample code that I used to achieve it  -  class PalindromeRecursive     public static void main String   args          Scanner sc new Scanner System in       System out println  Enter a string        String input sc next        System out println  is    input    a palindrome        isPalindrome input             public static  boolean isPalindrome String s          int low 0      int high s length  -1      while low lt high              if s charAt low   s charAt high         return false        isPalindrome s substring low   high--               return true

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Here I am pasting code for you   But  I would strongly suggest you to know how it works   from your question   you are totally unreadable     Try understanding this code  Read the comments from code  import java util Scanner  public class Palindromes        public static boolean isPal String s                if s length      0    s length      1                 if length  0 OR 1 then it is             return true           if s charAt 0     s charAt s length  -1                  check for first and last char of String                 if they are same then do the same thing for a substring                with first and last char removed  and carry on this                until you string completes or condition fails             return isPal s substring 1  s length  -1                if its not the case than string is not          return false             public static void main String  args                Scanner sc   new Scanner System in           System out println  type a word to check if its a palindrome or not            String x   sc nextLine            if isPal x               System out println x     is a palindrome            else             System out println x     is not a palindrome

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public class PlaindromeNumbers     int func1 int n        if n  1          return 1       return n func1 n-1      static boolean check false  int func int no         String a    no       String reverse   new StringBuffer a  reverse   toString         if a equals reverse                  if  a contains  0                         System out println  hey                check true              return Integer parseInt a                               else                 func no         if check  true                return 0               return 0          public static void main String   args           TODO code application logic here     Scanner in new Scanner System in       System out println  Enter testcase       int testcase in nextInt        while testcase gt 0            int a in nextInt         PlaindromeNumbers obj new PlaindromeNumbers           System out println obj func a           testcase--

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public class chkPalindrome   public static String isPalindrome String pal    if pal length      1    return pal    else   String tmp       tmp   tmp   pal charAt pal length  -1  isPalindrome pal substring 0 pal length  -1     return tmp             public static void main String   args             chkPalindrome hwObj   new chkPalindrome             String palind    MADAM           String retVal  hwObj isPalindrome palind         if retVal equals palind          System out println palind   is Palindrome           else        System out println palind   is Not Palindrome

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Here is a recursive method that will ignore specified characters   public static boolean isPal String rest  String ignore        int rLen   rest length        if  rLen  lt  2          return true      char first   rest charAt 0      char last   rest charAt rLen-1       boolean skip   ignore indexOf first     -1    ignore indexOf last     -1      return skip    first    last  amp  amp  isPal rest substring 1  rLen-1   ignore       Use it like this   isPal  Madam I m Adam  toLowerCase            isPal  A man  a plan  a canal  Panama  toLowerCase              It does not make sense to include case insensitivity in the recursive method since it only needs to be done once  unless you are not allowed to use the  toLowerCase   method

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Try this   package javaapplicationtest   public class Main        public static void main String   args             String source    mango           boolean isPalindrome   true             looping through the string and checking char by char from reverse         for int loop   0  loop  lt  source length    loop                           if  source charAt loop     source charAt source length  -loop-1                    isPalindrome   false                  break                                    if isPalindrome    false                System out println  Not a palindrome                        else              System out println  Pailndrome

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public static boolean isPalindrome String str        int len   str length        int i  j      j   len - 1      for  i   0  i  lt    len - 1  2  i                if  str charAt i     str charAt j         return false        j--            return true

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public static boolean isPalindrome String in      if in equals         in length    lt  2   return true     if in charAt 0  equalsIgnoreCase in charAt in length-1         return isPalindrome in substring 1 in length-2       else       return false       Maybe you need something like this  Not tested  I m not sure about string indexes  but it s a start point

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there s no code smaller than this   public static boolean palindrome String x       return  x charAt 0     x charAt x length  -1    amp  amp            x length   lt 4    palindrome x substring 1  x length  -1         if you want to check something   public static boolean palindrome String x       if x  null    x length    0           throw new IllegalArgumentException  Not a valid string               return  x charAt 0     x charAt x length  -1    amp  amp            x length   lt 4    palindrome x substring 1  x length  -1         LOL B-

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String source    liril   StringBuffer sb   new StringBuffer source   String r   sb reverse   toString    if  source equals r         System out println  Palindrome          else       System out println  Not a palindrome

[java] Creating a recursive method for Palindrome

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