[FIXED] How does StringBuilder reverse work in Java?

Issue

I am trying to solve this leetcode question https://leetcode.com/problems/palindrome-linked-list/ , but running into trouble with strings and string builders. For some reason, "12".equals"21" -> returns true.

I tried converting from string builder to string, and just using stringbuilder.

class Solution {
public boolean isPalindrome(ListNode head) {
    StringBuilder s = new StringBuilder();
    while (head != null) {
        s.append(head.val);
        head = head.next;
    }
    String a = s.reverse().toString(); 
    String b = s.toString(); 
    return a.equals(b);
}
}

It fails on "12" test case, and returns true.

Solution

StringBuilder reverse does not produce a new StringBuilder instance. It causes the underlying characters of the current StringBuilder to be reversed. So,

String a = s.reverse().toString(); 
String b = s.toString();

The second s.toString() is operating on the reversed StringBuilder.

you have to do

String original = s.toString(); 
String reversed = s.reverse().toString();
return original.equals(reversed);

Answered By - user7
Answer Checked By - Cary Denson (JavaFixing Admin)