LeetCode: Palindrome Linked List

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }

 Given a singly linked list, determine if it is a palindrome.

Example 1:

Input: 1->2
Output: false
Example 2:

Input: 1->2->2->1
Output: true
Follow up:
Could you do it in O(n) time and O(1) space?

 */
public class PalindromeLinkedList {
    public boolean isPalindrome(ListNode head) {
        if(head == null || head.next == null) {
            return true;
        }
     
        Stack<Integer> stack = new Stack<Integer>();
     
        ListNode fast = head;
        ListNode slow = head;

        while(fast != null && fast.next != null) {
            stack.push(slow.val);
            fast = fast.next.next;
            slow = slow.next;
        }
     
        if(fast != null) {
            slow = slow.next;
        }
     
        while(slow != null) {
            if(stack.pop() != slow.val) {
                return false;
            }

            slow = slow.next;
        }
     
        return true;
    }
}