Decagon·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Decagon software engineer interview had me implementing a Tic Tac Toe evaluator from scratch, starting brute-force and then optimizing. Pretty focused session, all algorithmic, no fluff.

Questions Asked (1)

Implement a Tic Tac Toe game-state evaluator: given a 3x3 board with X, O, and empty cells, determine the optimal outcome assuming both players play perfectly. Start with brute-force minimax, then optimize with memoization, and discuss state-space size and how board symmetries could reduce the search further.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I got the basic minimax down without too much trouble but fumbled a bit explaining the memoization step.

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AI HintsAI Generated

Suggested Approach

Start by clearly defining the game rules and the minimax algorithm, then implement a brute-force version. Next, optimize with memoization by caching board states, and finally analyze the state space size and discuss symmetry reductions. Throughout, emphasize the trade-offs between simplicity and efficiency.

Pro tip: Mention that the state space is small enough that even brute-force minimax is feasible, but memoization and symmetry reduction demonstrate deeper optimization skills. Also, note that the initial board state can be any valid configuration, not just empty.

1. Clarify rules and assumptions

Confirm that X goes first, players alternate, and the game ends when a player wins or the board is full. Assume optimal play from both sides.

2. Implement brute-force minimax

Write a recursive function that evaluates all possible moves, returning the optimal outcome for the current player. Base cases: win, loss, or draw.

3. Optimize with memoization

Use a hash map to cache results for board states, keyed by a string representation. This avoids recomputing identical subgames.

4. Analyze state space and symmetries

Calculate the total number of possible board states (3^9 = 19683) and valid game states. Discuss how board symmetries (rotations and reflections) can reduce the search space by up to 8x.

5. Discuss trade-offs and extensions

Compare time/space complexity of brute-force vs memoized vs symmetry-reduced. Mention that for Tic Tac Toe, even brute-force is fast, but these techniques scale to larger games.

Key Points to Mention

Minimax algorithm with recursion and base cases
Memoization using a hash map to cache board evaluations
State space size: 3^9 = 19683 possible boards, but only 5478 reachable valid states
Symmetry reduction: 8 symmetries (4 rotations, 2 reflections) can reduce states by up to 8x
Time complexity: O(b^d) for brute-force, where b is branching factor and d is depth
Trade-offs: memoization trades space for time; symmetry reduction adds complexity but reduces states

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.