1. /**
    2.  * Definition for singly-linked list.
    3.  * function ListNode(val) {
    4.  *     this.val = val;
    5.  *     this.next = null;
    6.  * }
    7.  */
    8. /**
    9.  * @param {ListNode} head
    10.  * @return {boolean}
    11.  */
    13. var isPalindrome = function (head) {
    14.     if(head == null || head.next == null){
    15.         return true;
    16.     }
    19.     let left = head;
    20.     let midNode = findMid(head);
    21.     let right = reverse(midNode);
    23.     // 开始比较
    24.     while(right != null){
    25.         if(left.val !== right.val){
    26.             return false;
    27.         }
    28.         left = left.next;
    29.         right = right.next;
    30.     }
    32.     return true;
    33. };
    35. // 双指针找到链表中间结点
    36. var findMid = function (head){
    37.     let fast, slow;
    38.     fast = slow = head;
    39.     while (fast != null && fast.next != null && slow != null) {
    40.         slow = slow.next;
    41.         fast = fast.next.next;
    42.     }
    43.     return slow;
    44. }
    46. // 翻转以head为头的链表
    47. var reverse = function (head) {
    48.     let pre = null, cur = head;
    49.     while(cur!=null){
    50.         let next = cur.next;
    51.         cur.next = pre;
    52.         pre = cur;
    53.         cur = next;
    54.     }
    55.     return pre;
    56. }