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.
Input: l1 = [2,4,3], l2 = [5,6,4]
Explanation: 342 + 465 = 807.
Input: l1 = , l2 = 
Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]