逆转链表

链表结构
/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

 1  /**
 2  * @param {ListNode} head
 3  * @return {ListNode}
 4  */
 5 var reverseList = function(head) {
 6         if(head === null){
 7             return null;
 8         }
 9         let newhead = null;
10         let tem = head;
11         while (tem != null){
12             let tt = tem.next;
13             tem.next = newhead;
14             newhead = tem;
15             tem = tt;
16         }
17         return newhead;
18 };