2.7 Implement a function to check if a linked list is a palindrome.

2.7 linkedListPalindrome

public boolean linkedListPalindrome(ListNode head) {
    if (head == null || head.next == null) {
        return true;
    }
    
    ListNode slow = head;
    ListNode fast = head;
    Stack<Integer> s = new Stack<Integer>();
    s.push(slow.val);
    
    while (fast.next != null && fast.next.next != null) {
        slow = slow.next;
        fast = fast.next.next;
        s.push(slow.val);
    }
    
    // The number of nodes is odd when fast is the last.
    if (fast.next == null) {
        s.pop();
    }
    
    slow = slow.next;
    while (slow != null) {
        if (s.pop() != slow.val) {
            return false;
        }
        slow = slow.next;
    }
    
    return true;
}