# 2. 两数相加

给出两个 非空 的链表用来表示两个非负的整数。其中，它们各自的位数是按照 逆序 的方式存储的，并且它们的每个节点只能存储 一位 数字。

如果，我们将这两个数相加起来，则会返回一个新的链表来表示它们的和。

您可以假设除了数字 0 之外，这两个数都不会以 0 开头。

> 示例：
>
> 输入：(2 -> 4 -> 3) + (5 -> 6 -> 4) 输出：7 -> 0 -> 8 原因：342 + 465 = 807

## 思路

按位相加，注意进位的处理。有进位的时候，也要再次循环。

## 实现

```javascript
/**
 * 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 p1 = l1
  let p2 = l2
  let carry = 0
  const dummy = new ListNode()
  let pointer = dummy
  while (p1 || p2 || carry) {
    const num1 = p1 ? p1.val : 0
    const num2 = p2 ? p2.val : 0
    const sum = num1 + num2 + carry
    if (sum > 9) {
      pointer.next = new ListNode(sum % 10)
      carry = 1
    } else {
      pointer.next = new ListNode(sum)
      carry = 0
    }
    if (p1) p1 = p1.next
    if (p2) p2 = p2.next
    pointer = pointer.next
  }
  return dummy.next
};
```


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