合并两个有序链表

将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

示例：

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

这道题思路比较简单，重点是熟悉链表的基本用法

/**
 * 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 === null) return l2;
    if (l2 === null) return l1;
    var head = null;
    var cur = null;
    while (l1 !== null || l2 !== null){
        if(l1 === null){
            cur = cut(cur, l2);
            return head || cur;
        }
        else if (l2 === null){
            cur = cut(cur,l1);
            return head||cur;   
        }
        else{
            if (l1.val<l2.val){
                if(cur === null){
                    head = l1;
                    cur = head;
                }
                else{
                    cur.next = l1;
                    cur = l1;
                }
                l1 = l1.next;
            }
            else{
                if (cur === null){
                    head = l2;
                    cur = head;
                }
                else{
                    cur.next = l2;
                    cur = l2;
                }
                l2 = l2.next;
            }
        }
    }
    return head;
};
function cut(merged,l){
    if (merged === null) return l;
    else{
        merged.next = l;
        return merged;
    }
}