/**
* Definition for singly-linked list.
* function ListNode(val, next) {
* this.val = (val===undefined ? 0 : val)
* this.next = (next===undefined ? null : next)
* }
*/
/**
* @param {ListNode} head
* @return {ListNode}
*/
var reverseList = function(head) {
let prev = null;
let cur = head;
while (cur !== null) {
const next = cur.next;
// 修改指针
cur.next = prev;
// 推进指针
prev = cur;
cur = next;
}
return prev;
};