To check if string is palindrome using recursion?

Question

There is something wrong with my code as one the testcase in my assignment is coming out wrong, giving me runtime error when I submit the code online. That testcase could be any String. I believe that everything is fine with the code as I have checked it manually for many testcases.

HERE IS THE CODE

public static boolean isStringPalindrome(String input) {
    if(input.length()==0 || input.length()==1)
        return true;

    int first = 0;
    int last = input.length()-1;

    if(input.charAt(first) != input.charAt(last))
        return false;

    String str="";
    for(int i=first+1;i<last;i++){
        str = str+input.charAt(i); 
    }
    boolean sa = isStringPalindrome(str);
    return sa; 
}

Sample Input

racecar

Output

true   

Sample Input

pablo

Output

false   

Answer 1

Your code appears to be overly complicated for recursively testing if the String is a palindrome. Something like,

public static boolean isStringPalindrome(String input) {
    if (input == null) {
        return false;
    } else if (input.isEmpty() || input.length() == 1) {
        return true;
    }
    int len = input.length() - 1;
    return input.charAt(0) == input.charAt(len) //
            && isStringPalindrome(input.substring(1, len));
}

Is recursive without embedding a for loop. Because if you can do that, you should do something like

public static boolean isStringPalindrome(String input) {
    if (input == null) {
        return false;
    } else if (input.isEmpty() || input.length() == 1) {
        return true;
    }
    int len = input.length();
    for (int i = 0; i <= len / 2; i++) {
        if (input.charAt(i) != input.charAt(len - 1 - i)) {
            return false;
        }
    }
    return true;
}

Answer 2

A simpler way to check for palindrome can be:

public static boolean isPalindrome(String s)
{   if (input == null)
        return false;
    else if(s.length() == 0 || s.length() == 1)
        return true;

    /* 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.
     */
    if(s.charAt(0) == s.charAt(s.length()-1))
        return isPalindrome(s.substring(1, s.length()-1));

    return false;
}

Update

You are getting runtime error(NZEC) which means non-zero exit code. It means your program is ending unexpectedly. I don't see any reason except that your program doesn't have a null check. Otherwise, I have gone through your code carefully, you are doing the same thing which I have suggested.