第21题：Merge Two Sorted Lists

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.

题目要求：合并两个已排序单链表。

编程语言：javascript

解法：算法复杂度为O(n)  思路：建立一个新的单链表，从头遍历两个单链表，将数值小的节点插入新的单链表中。

/** * 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) {        return mergeTwoLists(l1,l2);        function mergeTwoLists(list1,list2)    {        if(list1 === null) return list2;        if(list2 === null) return list1;                if(list1.val <= list2.val)        {            list1.next = mergeTwoLists(list1.next,list2);            return list1;        }else{            list2.next = mergeTwoLists(list1,list2.next);            return list2;        }    }};