[2] 两数相加

 1 /**
 2  * @param {ListNode} l1
 3  * @param {ListNode} l2
 4  * @return {ListNode}
 5  */
 6 var addTwoNumbers = function (l1, l2) {
 7   let addOne = 0
 8   let sum = new ListNode('0')
 9   let head = sum
10   while (addOne || l1 || l2) {
11     let val1 = l1 !== null ? l1.val : 0 // 优化点
12     let val2 = l2 !== null ? l2.val : 0 //优化点
13     let r1 = val1 + val2 + addOne
14     addOne = r1 >= 10 ? 1 : 0
15     sum.next = new ListNode(r1 % 10)
16     sum = sum.next
17     if (l1) l1 = l1.next
18     if (l2) l2 = l2.next
19   }
20   return head.next
21 };