Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.*

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 0 -> 8

var addTwoNumbers = function (l1, l2) {
  if (l1 && l2) {
    let val = l1.val + l2.val;
    let l = new ListNode(val % 10);

    if (!l1.next && !l2.next) {
      l.next = val > 9 ? new ListNode(1) : null;
      return l;
    }

    if (!l1.next) {
      l1.next = new ListNode(0);
    }

    if (!l2.next) {
      l2.next = new ListNode(0);
    }

    if (val > 9) {
      if (l1.next) {
        l1.next.val++;
      } else {
        l1.next = new ListNode(1);
      }
    }

    l.next = addTwoNumbers(l1.next, l2.next);
    return l;
  } else {
    return null;
  }
};

兩個 list 不一定會是等長，因此當沒有下一個節點的時候會新增一個假的節點，在下一回合檢查的時候

if (!l1.next && !l2.next)

為真即最後回合。