var reverseList = function(head) {
if (head == null || head.next == null) return head;
let last = reverseList(head.next);
head.next.next = head;
head.next = null;
return last;
};
function reverse(head) {
let prev = null, cur = head, nxt = head;
while (cur != null) {
nxt = cur.next;
cur.next = prev;
prev = cur;
cur = nxt;
}
return prev;
}