Merge two ordered lists into a new ordered list and return. The new linked list is composed of all nodes of two given linked lists.  

Example:

Input: 1 - > 2 - > 4, 1 - > 3 - > 4
 Output: 1 - > 1 - > 2 - > 3 - > 4 - > 4

/**
 * 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;
    console.log(l1 ,l1.val)
    if( l1.val < l2.val){
        l1.next = mergeTwoLists(l1.next,l2);
        return l1
    }else {
        l2.next = mergeTwoLists(l1,l2.next);
        return l2
    }

};

Before this, I wrote a version like this

/**
 * 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) {
    let newA = [],newB = [];

    if (l1.length >= l2.length) {
        newA = l1;
    }else {
        newA = l2;
    }
    newA.map((d,i)=>{
        if ( l1[i] == null) return newB.push(l2[i]);
        if ( l2[i] == null) return newB.push(l1[i]);
        if (l1[i] >= l2[i]){
            newB.push(l2[i],l1[i]);
        }else {
            newB.push(l1[i],l2[i]);
        }

    })
    return newB
};

The result shows that map is not a function, but it is normal in the console.

There is no way. I printed the input parameter separately and found that the input parameter is a chain structure.

It's not array, so it's always reported an error.

These times, I feel that leetcode's support for javascript is not very good, or it may be because it faces too many languages, and I have been engaged in the development of js, so I don't know some unused data types, but use the usual ideas to complete, there are many inexplicable errors.

However, through some reporting mistakes, I found my own narrow knowledge and limitations of thinking. Let's continue to work hard.

ps: sometimes when the idea encounters a bottleneck, search. The search results are basically java or python. Hope to meet more js gods to share.