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Databricks·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Databricks software engineer interview with a classic tic-tac-toe design problem, scaled up to n x n. Pretty standard coding round but the follow-up on optimization kept things interesting.

Questions Asked (1)

Q1

Design and implement a class that supports an n x n Tic-Tac-Toe game for two players. After each move, return which player won (or 0 if nobody has yet).

Algorithms & Data StructuresSystem Design
Author's notes

My first instinct was to just scan the whole board after every move, which works but felt lazy.

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AI HintsAI Generated

Suggested Approach

Clarify requirements and constraints, then design a class that maintains board state and efficiently checks for a win after each move. Implement the win check by tracking counts of marks per row, column, and diagonal for each player, updating them on each move and checking if any reaches n.

Pro tip: Mention that you can optimize the win check to O(1) per move by maintaining row, column, and diagonal counts for each player, and that this approach scales well for large n. Also, discuss handling invalid moves and edge cases like a full board with no winner.

1. Clarify requirements and constraints

Ask about board size, player representation, move validation, and what to return if the game ends in a draw. Confirm that after each move, you need to return the winning player or 0.

2. Design the class structure

Define a class with a constructor that initializes an n x n board and data structures to track counts. Include a move method that takes row, column, and player, validates the move, updates the board and counts, and returns the winner.

3. Implement efficient win detection

Maintain arrays for each player: row counts, column counts, and two diagonal counts. After a move, increment the relevant counts and check if any equals n. This gives O(1) time per move.

4. Handle edge cases and validation

Check for invalid moves (out of bounds, occupied cell, wrong player turn) and throw exceptions or return an error. Also, track the number of moves to detect a draw when the board is full.

5. Discuss complexity and test

Explain that the solution uses O(n^2) space for the board and O(n) additional space for counts, with O(1) time per move. Walk through a few test cases to verify correctness.

Key Points to Mention

  • Use separate count arrays for each player to track rows, columns, and diagonals.
  • Update counts incrementally after each move and check if any count reaches n.
  • Validate moves: ensure the cell is empty, within bounds, and it's the correct player's turn.
  • Track the total number of moves to detect a draw when the board is full.
  • Time complexity: O(1) per move for win detection; space complexity: O(n^2) for board and O(n) for counts.
  • Consider thread safety if the game might be accessed concurrently, though not required for basic implementation.

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