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.
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