Design a sudoku game
PremiumMachine codingMedium
- Backtracking
- Constraint checking
- Bitmasks
Quick answer
This machine coding interview question asks you to design a sudoku game. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Build a Sudoku game engine: a 9 × 9 grid where every row, column and 3 × 3 box must hold the digits 1 to 9 once. Players place and erase digits; the engine checks moves, gives hints and solves puzzles. The hard parts are fast rule checks and a solver that can also prove a puzzle has exactly one solution.
What must a sudoku game do?
- Load: load(puzzle) takes an 81-character string (digits for givens, . or 0 for blanks) and rejects a bad length, bad characters or clashing givens.
- Place and erase: place(r, c, d) and erase(r, c) return OK, CONFLICT with the clashing cells, or FIXED for a given. Strict mode rejects a clash; casual mode keeps and marks it.
How well must it do it?
- Cheap move checks: Keep row, column and box masks or digit counts so a move check is a few operations, updated on every place, erase, undo and redo.
- Solver speed: Solve any valid puzzle within 1 second by trying the empty cell with the fewest candidates first.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Backtracking
- Constraint checking
- Bitmasks
- Unique solution
- Undo redo
- Puzzle generation
How you’ll practise
- Requirements
- Entities and interfaces
- Core implementation
- Edge cases and tests
- Extensions
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any machine coding design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a machine coding question. You design a sudoku game and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team