old_contents: // Source : https://oj.leetcode.com/problems/merge-two-sorted-lists/
  // Author : Dean Shi
  // Date   : 2015-06-11
  
  /**********************************************************************************
   *
   * Merge two sorted linked lists and return it as a new list. The new list should be
   * made by splicing together the nodes of the first two lists.
   *
   **********************************************************************************/
  
  /**
   * 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) {
      let newList = null;
      
      if (l1 === null || l2 === null) {
          return l1 || l2
      }
      
      if (l1.val < l2.val) {
          (newList = new ListNode(l1.val)).next = mergeTwoLists(l1.next, l2)
      } else {
          (newList = new ListNode(l2.val)).next = mergeTwoLists(l1, l2.next)
      }
      
      return newList
  };
  
new_contents: // Source : https://oj.leetcode.com/problems/merge-two-sorted-lists/
  // Author : Dean Shi
  // Date   : 2015-06-11
  
  /**********************************************************************************
   *
   * Merge two sorted linked lists and return it as a new list. The new list should be
   * made by splicing together the nodes of the first two lists.
   *
   **********************************************************************************/
  
  /**
   * 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
      
      if (l1.val > l2.val) {
          l2.next = mergeTwoLists(l1, l2.next)
          return l2
      } else {
          l1.next = mergeTwoLists(l1.next, l2)
          return l1
      }
  };
  
subject: Update MergeTwoSortedList.js