Palindrome Linkedlist using recursion

Question

Indexing starts from 0. Input format: Linked list elements (separated by space and terminated by -1)

There is some runtime error on one hidden test case. main() is taking input and passing head .

public class Solution {
    static String s1="",s2="";
    public static boolean isPalindrome_2(LinkedListNode<Integer> head) {
        if (head != null) {
            s1 = s1 + head.data;
            isPalindrome_2(head.next);
            s2 = s2 + head.data;
        }
        if (s1.equals(s2))
            return true;
        return false;
    }
}

How the algorithm is supposed to work: s1 will store the string containing all data. s2 will store data in reverse manner since it is after recursive function. Then the strings can be compared.

Answer 1

Since you have not provided a reproducible example, I have not been able to run your code and verify any hypotheses. So this is from inspection alone.

1. If the numbers in the list ar 1 and 11, it is not a palindrome. Yet your string s1 and s2 should both become 111 , that is equal, so your method should return true .
2. You are not testing for the -1 at the end. Shouldn't you?

BTW I don't see any indexing and I don't see any space between elements.

I also have not understood how a runtime error can occur, sorry. Again, running your code might have given me a greater chance.