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


var reverseList = function(head) {
  var previousNode = null;
  while (head) {
    var nextNode = head.next;
    head.next = previousNode;
    previousNode = head;
    head = nextNode;
  }
  return previousNode;
};