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 {ListNode}
    11.  */
    12. var reverseList = function(head) {
    13.     let cur = null,
    14.         tail = null
    16.     while(head !== null) {
    17.         tail = head
    18.         head = head.next
    19.         tail.next = cur
    20.         cur = tail
    21.     }
    22.     return tail
    23. };