[LC 234.回文链表]

快慢指针+反转链表

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public boolean isPalindrome(ListNode head) {
        if (head == null || head.next == null)
            return true;
        ListNode slow,fast;
        slow = fast = head;
        while (fast != null && fast.next!=null){
            slow = slow.next;
            fast = fast.next.next;
        }
        if (fast!=null)
            slow = slow.next;
        ListNode left = head;
        ListNode right = reverse(slow);
        while (right != null){
            if(left.val != right.val)
                return false;
            left = left.next;
            right = right.next;
        }
        return true;

    }
    public ListNode reverse(ListNode a){
        ListNode pre,cur,nxt;
        pre = null;
        cur = nxt = a;
        while (cur != null){
            nxt = cur.next;
            cur.next = pre;
            pre = cur;
            cur = nxt;
        }
        return pre;
    }
}