Merge k Sorted Lists

Problem statement

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order.

Merge all the linked-lists into one sorted linked-list and return it.

Example 1:

Input: lists = [[1,4,5],[1,3,4],[2,6]]Output: [1,1,2,3,4,4,5,6]Explanation: The linked-lists are:[  1->4->5,  1->3->4,  2->6]merging them into one sorted list:1->1->2->3->4->4->5->6

Example 2:

Input: lists = []Output: []

Example 3:

Input: lists = [[]]Output: []

Constraints:

• k == lists.length
• 0 <= k <= 104
• 0 <= lists[i].length <= 500
• -104 <= lists[i][j] <= 104
• lists[i] is sorted in ascending order.
• The sum of lists[i].length will not exceed 104.

My solution

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode[]} lists
 * @return {ListNode}
 */
var mergeKLists = function(lists) {
    function merge(left, right) {
        const newNode = new ListNode();
        let currentNode = newNode;
        
        while(left && right) {
            // console.log(left, right)
            // console.log("currentNode", currentNode)
            // console.log("left.val", left.val)
            // console.log("right.val", right.val)
            if (left.val < right.val) {
                // console.log("LEFT ASSIGN")
                currentNode.next = left;
                left = left.next;
            } else {
                // console.log("RIGHT ASSIGN")
                currentNode.next = right;
                right = right.next
            }
            currentNode = currentNode.next;
        }
        
        if (left) {
            currentNode.next = left;
        } else if (right) {
            currentNode.next = right;
        }
        // console.log("currentNode", currentNode.next)
        return newNode.next;
    }
    
    let root = lists[0];
    
    for (let i = 1; i < lists.length; i++) {
        // console.log("=====")
        root = merge(root, lists[i])
    }
    
    return root || null;
};