234. 回文链表 (JAVA)

/**
 * 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) {
        ListNode fast = head, slow = head, pre = null, tmp;
        while(fast.next!=null) {
            fast = fast.next.next;
            tmp = slow.next;
            slow.next = pre;
            pre = slow;
            slow = tmp;
            if(fast == null){ //双数
                return check(pre, slow);
            }
        }

        //单数
        return check(pre, slow.next);
    }

    private boolean check(ListNode l1, ListNode l2){
        if(l1 == null) return true;
        if(l1.val != l2.val) return false;
        return check(l1.next, l2.next);
    }
}