1. /**
    2.  * Definition for singly-linked list.
    3.  * function ListNode(val, next) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.next = (next===undefined ? null : next)
    6.  * }
    7.  */
    8. /**
    9.  * @param {ListNode} head
    10.  * @return {boolean}
    11.  */
    12. var isPalindrome = function (head) {
    13.     if (!head.next) return true
    15.     let slow = head,
    16.         fast = head,
    17.         cur = null,
    18.         conut = 0
    20.     // 快慢指针,step = 2,找到中间节点
    21.     while (fast && fast.next) {
    22.         conut++
    23.         fast = fast.next.next
    24.         slow = slow.next
    25.     }
    27.     let middle = null
    28.     // 反转前半段链表
    29.     for (let i = 0; i < conut; i++) {
    30.         middle = head
    31.         head = head.next
    32.         middle.next = cur
    33.         cur = middle
    34.     }
    37.     // 如果fast不为null，说明长度是奇数，slow需要向后移一位
    38.     if(fast) {
    39.         slow = slow.next
    40.     }
    42.     while (slow && middle) {
    43.         if (middle.val !== slow.val) return false
    44.         slow = slow.next
    45.         middle = middle.next
    46.     }
    48.     return true
    49. };