Return all strings formed by mapping each digit from 2 through 9 to its telephone letters.
A good solution should
- State the invariant or decision rule that makes the approach correct.
- Explain time and space costs in terms of input size and required output.
- Handle empty, minimal, duplicate, and boundary-shaped inputs.
Reasoning prompt
The output has a product-sized number of combinations. Choose one mapped letter per position and make empty input behavior explicit.
Use the linked judge for the canonical challenge. The examples above are compact TRT checks; create additional tests around the boundary most likely to break your invariant.