LC 21. Merge Two Sorted Listed

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
var mergeTwoLists = function(list1, list2) {
  if (list1 === null) {
    return list2
  } else if (list2 === null) {
    return list1
  } else if (list1.val < list2.val) {
    list1.next = mergeTwoLists(list1.next, list2)
    return list1
  } else {
    list2.next = mergeTwoLists(list1, list2.next)
    return list2
  }
};