### Title

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 contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

### Example

``````Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.``````

### Analyze

``````// Keyword: single linked list

/**
* function ListNode(val) {
*     this.val = val;
*     this.next = null;
* }
*/
/**
* @param {ListNode} l1
* @param {ListNode} l2
* @return {ListNode}
*/
var addTwoNumbers = function(l1, l2) {
let flag = 0
temp = null
while (l1 || l2) {
let sum = flag
if (l1) {
sum += l1.val
l1 = l1.next
}

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

const list = new ListNode(sum % 10)
temp = list
} else {
temp.next = list
temp = list
}

// 处理进位
flag = 0
if (sum >= 10) {
flag = 1
}
}

if (flag === 1) {
const result = new ListNode(1)
temp.next = result
temp = result
}