text: /**
   * Definition for singly-linked list.
   * function ListNode(val) {
   *     this.val = val;
   *     this.next = null;
   * }
   */
  /**
   * @param {ListNode} l1
   * @param {ListNode} l2
   * @return {ListNode}
   */
  var mergeTwoLists = function(l1, l2) {
      if (!l1) return l2;
      if (!l2) return l1;
      let head = null;
      if (l1.val <= l2.val) {
          head = l1;
          l1 = l1.next;
      } else {
          head = l2;
          l2 = l2.next;
      }
      let cur = head;
      while (!!l1 && !!l2) {
          if (l1.val <= l2.val) {
              cur.next = l1;
              l1 = l1.next;
              cur = cur.next;
          } else {
              cur.next = l2;
              l2 = l2.next;
              cur = cur.next;
          }
      }
      cur.next = !!l1 ? l1 : l2;
      return head;
  };