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:
Example 2:
Example 3:
Constraints:
[1, 100]
.Node.val
<= 9Follow up: Could you solve it without reversing the input lists?