 ## Challenge Statement

• 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.
• This challenge corresponds to LeetCode #2.

### 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.

### Example 1:

Input: `l1 = [2, 4, 3]`, `l2 = [5, 6, 4]`

Output: `[7, 0, 8]`

Explanation: `342 + 465 = 807`

### Example 2:

Input: `l1 = `, `l2 = `

Output: ``

### 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]`

## Solution

Below is my solution and some test cases. This solution has a linear time complexity O(n) and a constant space complexity O(1), where n is the length of the input list.