21-merge-two-sorted-lists.js

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */

const mergeTwoLists = function(l1, l2) {
  if (l1 === null) return l2;
  if (l2 === null) return l1;
  if (l1.val < l2.val) {
    l1.next = mergeTwoLists(l1.next, l2);
    return l1;
  } else {
    l2.next = mergeTwoLists(l1, l2.next);
    return l2;
  }
};

// another

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
const mergeTwoLists = function(l1, l2) {
  const dummy = new ListNode()
  let cur = dummy
  while(l1 && l2) {
    if(l1.val < l2.val) {
      cur.next = new ListNode(l1.val)
      l1 = l1.next
    } else {
      cur.next = new ListNode(l2.val)
      l2 = l2.next
    }
    
    cur = cur.next
  }
  if(l1) cur.next = l1
  if(l2) cur.next = l2
  
  return dummy.next
};