Add Two Forward-Order Lists

Implement a function that adds two non-negative integers represented by singly linked lists. - Each node stores a single digit. - The digits are stored in **forward order**, so the head contains the most significant digit. - The input lists do not have leading zeros unless the number itself is zero. - Return the sum as a new linked list in **forward order**. You should define the `ListNode` structure yourself and handle common edge cases such as different list lengths and a final carry. Example: - `7 -> 2 -> 4 -> 3` represents 7243 - `5 -> 6 -> 4` represents 564 - Output: `7 -> 8 -> 0 -> 7` because 7243 + 564 = 7807 Follow-up: Can you solve it **without reversing the input lists**, if extra space is allowed?