[2] 两数相加

/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function (l1, l2) {
  let addOne = 0
  let sum = new ListNode('0')
  let head = sum
  while (addOne || l1 || l2) {
    let val1 = l1 !== null ? l1.val : 0 // 优化点
    let val2 = l2 !== null ? l2.val : 0 //优化点
    let r1 = val1 + val2 + addOne
    addOne = r1 >= 10 ? 1 : 0
    sum.next = new ListNode(r1 % 10)
    sum = sum.next
    if (l1) l1 = l1.next
    if (l2) l2 = l2.next
  }
  return head.next
};