Palindrome Linked List Leetcode

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?

 

这个follow up要求O(n)的时间和O(1)的空间，可以先reverse一半，然后再对比。只是reverse的时候要考虑奇数个还是偶数个。如果是奇数个的话，就跳过最中间的，从下一个开始reverse。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isPalindrome(ListNode head) {
        ListNode runner = head;
        ListNode walker = head;
        while (runner != null && runner.next != null) {
            runner = runner.next.next;
            walker = walker.next;
        }
        ListNode half = runner == null ? reverse(walker): reverse(walker.next);
        while (half != null) {
            if (head.val != half.val) {
                return false;
            }
            half = half.next;
            head = head.next;
        }
        return true;
    }
    private ListNode reverse(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode node = head.next;
        ListNode newHead = reverse(node);
        node.next = head;
        head.next = null;
        return newHead;
    }
}