# 2. Add Two Numbers (Medium)

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.

## Example 1:

![Add two number](assets/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]
```

## Constraints:

- The number of nodes in each linked list is in the range [1, 100].
- 0 <= Node.val <= 9
- It is guaranteed that the list represents a number that does not have leading
  zeros.