Microsoft·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jul 2026

Summary

Microsoft SWE interview with a design-your-own Tic-Tac-Toe class problem. Pretty classic but the O(1) constraint is where things get interesting and where I almost fumbled it.

Questions Asked (1)

Design and implement a TicTacToe class for an n×n board. It should support a move() method that takes a row, column, and player number, and returns 0 if no one has won yet, or the winning player's number if that move ends the game. The solution needs to run in O(1) time per move and O(n) space.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

My first instinct was to just scan the whole row and column after each move, which works but falls apart at n=10^4.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints, then propose a solution using row, column, and diagonal counters to track each player's progress. Explain how each move updates these counters in O(1) time and how a win is detected by checking if any counter reaches n or -n.

Pro tip: Mention that you would use separate arrays for each player or a single array with signed values to distinguish players, and emphasize that this approach avoids scanning the board, achieving true O(1) per move.

1. Clarify Requirements

Confirm the board size n, the player numbers (e.g., 1 and 2), and the win condition (n in a row, column, or diagonal). Ask if moves are guaranteed to be valid.

2. Design Data Structures

Propose maintaining arrays for row counts, column counts, and two diagonal counts. Use signed integers to represent each player's progress, or separate arrays per player.

3. Implement move() Logic

For each move, update the corresponding row, column, and diagonal counters (if applicable) by adding +1 for player 1 and -1 for player 2. Then check if any updated counter equals n or -n.

4. Analyze Complexity

Explain that each move updates a constant number of counters and performs constant-time checks, achieving O(1) time per move. Space is O(n) due to the arrays of size n.

5. Discuss Edge Cases and Trade-offs

Address invalid moves, multiple wins, and the possibility of a draw. Mention that this approach assumes valid moves and does not track the board state, which is acceptable for the given requirements.

Key Points to Mention

O(1) time per move by updating counters instead of scanning the board
O(n) space using arrays of size n for rows, columns, and diagonals
Use of signed integers to represent both players in a single array
Win detection by checking if any counter reaches n or -n
Handling of diagonal updates only when row == col or row + col == n-1
Assumption that moves are valid and players alternate, but code can be adapted

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