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?

Answer 1

there's no code smaller than this:

public static boolean palindrome(String x){
    return (x.charAt(0) == x.charAt(x.length()-1)) && 
        (x.length()<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)) && 
        (x.length()<4 || palindrome(x.substring(1, x.length()-1)));
}

LOL B-]

Answer 2

public static boolean isPalindrome(String str)
{
    int len = str.length();
    int i, j;
    j = len - 1;
    for (i = 0; i <= (len - 1)/2; i++)
    {
      if (str.charAt(i) != str.charAt(j))
      return false;
      j--;
    }
    return true;
} 

Answer 3

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>0)
   {
     int a=in.nextInt();
     PlaindromeNumbers obj=new PlaindromeNumbers();
       System.out.println(obj.func(a));
       testcase--;
   }
}

}

Answer 4

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...");
}

Answer 5

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 <= 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 < 2 ||
           str.charAt(start) == str.charAt(stop) &&
           check(str, start + 1, stop - 1);
}

Answer 6

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 < 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");

    }

}