合并两个有序链表

输入：l1 = [1,2,4], l2 = [1,3,4]
输出：[1,1,2,3,4,4]

输入：l1 = [], l2 = []
输出：[]

输入：l1 = [], l2 = [0]
输出：[0]

/**
 * 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 listNode = new ListNode(-1)
    let prev = listNode
    while (l1 != null && l2 != null) {
        if (l1.val <= l2.val) {
            prev.next = l1
            l1 = l1.next
        } else {
            prev.next = l2
            l2 = l2.next
        }
        prev = prev.next
    }
    prev.next = l1 === null ? l2 : l1
   
    return listNode.next
};

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