2.js

/** ------------------------------------------------------------------------------
 * 
 * 2. Add Two Numbers
 * Topics: Linked List
 * https://leetcode.com/problems/add-two-numbers/description/?envType=problem-list-v2&envId=linked-list
 * 
 ------------------------------------------------------------------------------ */
/**
 * 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 list = new ListNode(0)
  let head = list
  let sum = 0,
    carry = 0

  while (l1 !== null || l2 !== null || sum > 0) {
    if (l1 !== null) {
      sum += l1.val
      l1 = l1.next
    }

    if (l2 !== null) {
      sum += l2.val
      l2 = l2.next
    }

    if (sum >= 10) {
      carry = 1
      sum -= 10
    }

    head.next = new ListNode(sum)
    head = head.next

    sum = carry
    carry = 0
  }
  return list.next
}