text: /**
   * 2. Add Two Numbers
   * https://leetcode.com/problems/add-two-numbers/
   * Difficulty: Medium
   *
   * 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 contains a single
   * digit. Add the two numbers and return the sum as a linked list.
   *
   * You may assume the two numbers do not contain any leading zero, except the number 0 itself.
   */
  
  /**
   * Definition for singly-linked list.
   * function ListNode(val, next) {
   *     this.val = (val===undefined ? 0 : val)
   *     this.next = (next===undefined ? null : next)
   * }
   */
  /**
   * @param {ListNode} l1
   * @param {ListNode} l2
   * @return {ListNode}
   */
  var addTwoNumbers = function(l1, l2) {
    const result = new ListNode();
    let tail = result;
    let carry = 0;
  
    while (l1 || l2 || carry) {
      const v1 = l1 ? l1.val : 0;
      const v2 = l2 ? l2.val : 0;
      const v = v1 + v2 + carry;
  
      tail.next = new ListNode(v % 10);
      tail = tail.next;
      carry = v >= 10 ? 1 : 0;
      l1 = l1 && l1.next;
      l2 = l2 && l2.next;
    }
  
    return result.next;
  };
  