234. Palindrome Linked List

题目：

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?

链接： http://leetcode.com/problems/palindrome-linked-list/

2/28/2017

参考别人的答案，因为一开始不知道可不可以改变list的结构。

public class Solution {
    public boolean isPalindrome(ListNode head) {
        if (head == null || head.next == null) return true;
        ListNode mid = findMidNode(head);
        ListNode ret = reverseList(mid);

        while(ret != null) {
            if (head.val != ret.val) return false;
            head = head.next;
            ret = ret.next;
        }
        return true;
    }
    private ListNode findMidNode(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;

        while (fast != null) {
            fast = fast.next;
            if (fast != null) {
                fast = fast.next;
                slow = slow.next;
            }
        }
        return slow;
    }
    private ListNode reverseList(ListNode head) {
        ListNode dummy = new ListNode(0);
        ListNode tmp;
        while (head != null) {
            tmp = head.next;
            head.next = dummy.next;
            dummy.next = head;
            head = tmp;
        }
        return dummy.next;
    }
}