反转链表

class ListNode {
    constructor(val, next) {
        this.val = val;
        this.next = (next === undefined ? null : next);
    }
}
let node5 = new ListNode(5, null);
let node4 = new ListNode(4, node5);
let node3 = new ListNode(3, node4);
let node2 = new ListNode(2, node3);
let node1 = new ListNode(1, node2);
var reverseList = function(head) {
    if(head == null || head.next == null)   return head;
    var last = reverseList(head.next);
    head.next.next = head;
    head.next = null;
    return last;
};
var p = reverseList(node1);
var ans = [];
while(p != null) {
    ans.push(p.val);
    p = p.next;
}
console.log(ans);