输入两个递增排序的链表，合并这两个链表并使新链表中的节点仍然是递增排序的。

示例1：

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * 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}
 */
var mergeTwoLists = function(l1, l2) {
    let res = new ListNode(0, null)
    let cur = res

    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
      }
    }

    if(l1) {
        cur.next = l1
    }
    if(l2) {
        cur.next = l2
    }

    return res.next
};

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/he-bing-liang-ge-pai-xu-de-lian-biao-lcof