/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
let reverseList = function (head) {
  let prev = null
  let cur = head

  while (cur) {
    let next = cur.next
    cur.next = prev
    prev = cur
    cur = next
  }

  return prev
}