mergeKLists = function(lists) {
    let previous = null;

    for (let i = 0; i < lists.length; i++) {
        previous = mergeTwoLists(previous, lists[i]);
    }

    return previous;
}
https://leetcode.com/problems/merge-k-sorted-lists/
Time O(N) | Space O(N)
@param {ListNode[]} lists
@return {ListNode}