The reverse storage is actually a gift because you can traverse both lists from head to tail and that already gives you the digits in addition order, least significant first.
Start by clarifying the problem constraints and edge cases, then propose a two-pointer traversal that adds digits while carrying over, reusing the input lists to achieve O(1) extra space. Emphasize that the reverse order simplifies addition, and discuss how to handle carry propagation and list length differences.
Pro tip: Mention that you can reuse the longer input list to store the result, avoiding extra space, but be prepared to discuss the trade-off of mutating inputs. Also, explicitly handle the final carry by appending a new node if needed.
Ask about input constraints (e.g., non-negative integers, leading zeros, empty lists) and confirm that the output should be a new list or if reusing inputs is acceptable. Discuss time and space complexity expectations.
Explain that you will traverse both lists simultaneously, summing corresponding digits plus any carry, and building the result list in reverse order. Use a dummy head to simplify list construction.
Describe how to continue traversal if one list is longer, and how to propagate the carry. Ensure that after both lists are exhausted, any remaining carry is added as a new node.
Propose reusing the nodes of the longer input list to store the result, thus achieving O(1) extra space (excluding the output). Discuss the trade-off of mutating the input lists.
State that time complexity is O(max(n, m)) and space is O(1) extra. Walk through a few test cases, including different lengths and carry propagation.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.