var addTwoNumbers = function (l1, l2) {
  const stack1 = [],
    stack2 = [];
  while (l1 !== null) {
    stack1.push(l1.val);
    l1 = l1.next;
  }
  while (l2 !== null) {
    stack2.push(l2.val);
    l2 = l2.next;
  }

  let curr = null,
    carry = 0;

  while (stack1.length !== 0 || stack2.length !== 0) {
    let sum = 0;
    if (stack1.length !== 0) {
      sum += stack1.pop();
    }
    if (stack2.length !== 0) {
      sum += stack2.pop();
    }
    sum += carry;

    const node = new ListNode(sum % 10);
    carry = Math.floor(sum / 10);

    node.next = curr;
    curr = node;
  }

  if (carry !== 0) {
    const node = new ListNode(carry);
    node.next = curr;
    curr = node;
  }

  return curr;
};