addTwoNumbers = function(l1, l2) {
    let sentinel = tail = new ListNode();

    return add(l1, l2, tail, sentinel);       /* Time O(MAX(N, M)) | Space O(MAX(N, M)) */
}
https://leetcode.com/problems/add-two-numbers/
Time O(MAX(N, M)) | Space O(MAX(N, M))
@param {ListNode} l1
@param {ListNode} l2
@return {ListNode}