PalindromeLinkedList.java

import java.util.Stack;

public class PalindromeLinkedList {
    public static void main(String[] args) {
        PalindromeLinkedList tester = new PalindromeLinkedList();
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(2);
        head.next.next.next.next = new ListNode(1);
        //head.next.next.next.next.next = new ListNode(1);
        boolean test = tester.isPalindrome(head);
        System.out.print(test);
    }

    public boolean isPalindrome(ListNode head) {
        Stack<Integer> stack = new Stack<>();
        ListNode slow = head;
        ListNode fast = head;
        while (fast.next != null && fast.next.next != null) {
            stack.push(slow.val);
            slow = slow.next;
            fast = fast.next.next;
        }
        if (fast.next != null) {
            stack.push(slow.val);
        }
        slow = slow.next;
        while (slow != null) {
            if (stack.pop() != slow.val) {
                return false;
            }
            slow = slow.next;
        }
        return true;
    }
}

class ListNode {
    int val;
    ListNode next;
    public ListNode(int val) {
        this.val = val;
    }
    public ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }
}