Is a Singly Linked List Palindrome? JAVA

Question

It is the 234th question on LeetCode. The requirement for this task is to do it in O(n) time and O(1) space. Definition for singly-linked list:

public class ListNode {
  int val;
  ListNode next;
  ListNode(int x) { val = x; }

}

If I write the function like this:

public class Solution {
public boolean isPalindrome(ListNode head) {
    List<Integer> a = new ArrayList<>();
    while(head!=null){
        a.add(head.val);
        head = head.next;
    }
    for(int i=0, j=a.size()-1-i; i<j;){
        if(a.get(i) != a.get(j)){
            return false;
        }
        i++;
        j--;
    }
    return true;
}

}

This will return false if I try [128, 128], but it will return true if I try [127, 127]. I really do not know why. Could anybody help me?

Answer 1

You're decrementing your loop counter but comparing it to your position counter. In other words, while i<j will only go through half the list.

so,

for( int i=0, j=a.size()-1; i<j ; ++i,--j ) {
    if ( a.get(i) != a.get(j) ) return false;
}
return true;

or, a little more obfuscated.

int i=0, j=a.size()-1;
while ( j > i && s[i] == s[j] ) { ++i; --j; }
return j<=i;