You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first 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:

    Input: l1 = [7,2,4,3], l2 = [5,6,4]
    Output: [7,8,0,7]

Example 2:

    Input: l1 = [2,4,3], l2 = [5,6,4]
    Output: [8,0,7]

Example 3:

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

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.

Follow up: Could you solve it without reversing the input lists?