# 2. Add Two Numbers

### Leetcode

#### <https://leetcode.com/problems/add-two-numbers/>

### 題目

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 contains a single digit. Add the two numbers and return the sum as a linked list.

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

&#x20;

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/10/02/addtwonumber1.jpg)

```
Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.
```

**Example 2:**

```
Input: l1 = [0], l2 = [0]
Output: [0]
```

**Example 3:**

```
Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]
```

### 解答 <a href="#ti-jie" id="ti-jie"></a>

* **方法一**

```
var addTwoNumbers = function(l1, l2) {
    let arr = new ListNode(0);
    const head = arr;
    
    while(l1 || l2) {
        const number = (l1?.val || 0) + (l2?.val || 0);
        // 原本前一個進位的(arr.val) + 餘數
        const val = arr.val + number % 10;
        arr.val = val % 10;
        const next = Math.floor(number / 10) + Math.floor(val / 10) ;
        
        if(l1 !== null) l1 = l1.next;
        if(l2 !== null) l2 = l2.next;
        // next: 最尾端 next 有進位的狀況
        if(next || l1 || l2) {
            arr.next = new ListNode(next);
            arr = arr.next;
        }
    }
    return head;
};
```

Runtime: 124 ms, faster than 79.86% of JavaScript online submissions for Add Two Numbers.

Memory Usage: 43.9 MB, less than 88.11% of JavaScript online submissions for Add Two Numbers.
