1. /**
    2.  * Definition for singly-linked list.
    3.  * function ListNode(val, next) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.next = (next===undefined ? null : next)
    6.  * }
    7.  */
    8. /**
    9.  * @param {ListNode} l1
    10.  * @param {ListNode} l2
    11.  * @return {ListNode}
    12.  */
    13. var addTwoNumbers = function (l1, l2) {
    14.   let res = new ListNode(),
    15.       head = res,
    16.       cur1 = 0,
    17.       cur2 = 0,
    18.       next = 0
    20.   while (l1 || l2 || next) {
    21.     cur1 = l1 && l1.val || 0
    22.     cur2 = l2 && l2.val || 0
    23.     l1 = l1 && l1.next
    24.     l2 = l2 && l2.next
    26.     let sum = (cur1 + cur2 + next) % 10
    27.     next =  Math.floor((cur1 + cur2 + next) / 10)
    29.     res.next = new ListNode(sum)
    30.     res = res.next
    31.   }
    33.   return head.next
    34. };