leetcode 234: Palindrome Linked List

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?

[思路]

先分成大小同样(可能长度差1) 两部分,   reverse一个list. 再比較.

[CODE]

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
 
 // 1 2 3 2 1; 1 2 3 3 2 1;
public class Solution {
    public boolean isPalindrome(ListNode head) {
        //input check  abcba abccba
        if(head==null || head.next==null) return true;
        
        ListNode middle = partition(head);
        middle = reverse(middle);
        
        while(head!=null && middle!=null) {
            if(head.val != middle.val) return false;
            head = head.next;
            middle = middle.next;
        }
        return true;
    }
    private ListNode partition(ListNode head) {
        ListNode p = head;
        while(p.next!=null && p.next.next!=null) {
            p = p.next.next;
            head = head.next;
        }
        
        p = head.next;
        head.next = null;
        return p;
    }
    private ListNode reverse(ListNode head) {
        if(head==null || head.next==null) return head;
        ListNode pre = head;
        ListNode cur = head.next;
        pre.next = null;
        ListNode nxt = null;
        
        while(cur!=null) {
            nxt = cur.next;
            cur.next = pre;
            pre = cur;
            cur = nxt;
        }
        return pre;
    }
}