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:
first_list = [2,4,3]
second_list = [5,6,4]
Output:
[7,0,8]
Explanation: 342 + 465 = 807.
Input:
first_list = [0]
second_list = [0]
Output:
[0]
Input:
first_list = [9,9,9,9,9,9,9]
second_list = [9,9,9,9]
Output:
[8,9,9,9,0,0,0,1]
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.