### 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 -> 8Explanation: 342 + 465 = 807.

### Analyze

// Keyword: single linked list
/** * Definition for singly-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  let head = null,      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)    if (head === null) {      head = list      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  }  return head}

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