Examples

Input: 4->5->6
Output: 4->5->7
Explanation: 456 + 1 = 457

Input: 9->9->9
Output: 1->0->0->0
Explanation: 999 + 1 = 1000

Disclaimer: Here is the practice link to help you assess your knowledge better. It's highly recommend trying to solve it before looking at the solution.

Iterative approach

Algorithm

Since the least significant digit is at the end of the list, we can’t process it directly using a single traversal. Therefore, to perform addition from the end, we need to reverse the list.

Reverse the list and start from the head and add 1. Carry it forward as needed.
If a carry remains after the last node, add a new node with value 1.
Reverse the list again to restore the original order and return the head of the modified list.