234 Palindrome Linked List

234. Palindrome Linked List

1. Question

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

Follow up: Could you do it in O(n) time and O(1) space?

2. Implementation

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isPalindrome(ListNode head) {
        if (head == null || head.next == null) {
            return true;
        }

        ListNode fast = head, slow = head;

        while (fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }

        slow = reverseList(slow.next);
        fast = head;

        while (slow != null) {
            if (fast.val != slow.val) {
                return false;
            }
            fast = fast.next;
            slow = slow.next;
        }
        return true;
    }

    public ListNode reverseList(ListNode curNode) {
        ListNode preNode = null, nextNode = null;

        while (curNode != null) {
            nextNode = curNode.next;
            curNode.next = preNode;
            preNode = curNode;
            curNode = nextNode;
        }
        return preNode;
    }
}

3. Time & Space Complexity

时间复杂度O(n), 空间复杂度O(1)