Refactor: simplify validation and improve backtracking cleanup (#7258)

### Summary
This PR makes small readability and maintainability improvements to the algorithm implementation.

### Changes
- Removed a redundant `n < 0` validation check since the method contract already ensures valid `n`
- Replaced `current.remove(current.size() - 1)` with `current.removeLast()` to better express backtracking intent

### Rationale
- Simplifies input validation without changing behavior
- Uses the `Deque` API to make the backtracking step clearer and less error-prone

### Impact
- No change in algorithm logic or time/space complexity
- Output remains identical

Co-authored-by: Swati Vusurumarthi <swativs869@gmail.com>