Clarify the problem constraints and edge cases, then walk through a two-pointer iterative solution that splices nodes in place. Emphasize O(n+m) time and O(1) space, and discuss how to handle duplicates and stability.
Pro tip: Use a dummy head node to simplify edge cases and avoid special-casing the first node. Also, explicitly state that you are reusing the original nodes (no new allocations) to show awareness of memory efficiency.
Ask about input constraints (e.g., empty lists, duplicate values, sorted order) and whether modifying the original lists is acceptable. Confirm the expected return type.
Explain that you will maintain pointers to the current node in each list, compare their values, and append the smaller node to the merged list, advancing that pointer.
Describe using a dummy head to simplify list construction, and cover cases where one list is exhausted by linking the remainder of the other list.
State that time complexity is O(n+m) and space is O(1) since nodes are spliced in place. Mention that the merge is stable if you consistently choose the node from the first list when values are equal.
Walk through a simple example (e.g., 1->2->4 and 1->3->4) to verify correctness, and mention testing edge cases like empty lists or one list being longer.
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