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Build a Tic-Tac-Toe App | Whatnot Coding Question

Build a Tic-Tac-Toe App

Company: Whatnot

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: medium

Interview Round: Technical Screen

Implement a simple Tic-Tac-Toe mobile app that can be completed in a 45-minute coding interview. Requirements: - Display a 3x3 board. - Support two local players, `X` and `O`, taking alternating turns. - A player can place a mark only in an empty cell. - After every move, detect whether either player has won by completing a row, column, or diagonal. - Detect a draw when the board is full and there is no winner. - Prevent additional moves after the game is over. - Show the current game status: current player, winner, or draw. - Provide a reset button to start a new game. Focus on clean state management, readable UI code, and correctness rather than visual polish.

Quick Answer: This question evaluates proficiency in interactive UI development and stateful game logic, including turn management, board state representation, win/draw detection, input validation, and reset flow.

Implement the core game logic behind a simple Tic-Tac-Toe mobile app. Instead of building the UI, you are given a sequence of commands representing player taps and reset actions. Simulate the app exactly as it should behave on a 3x3 board. Rules: - The board starts empty. - Player X always goes first. - Players alternate turns only after a successful move. - A player may place a mark only in an empty cell. - After each successful move, check whether that player completed a row, column, or diagonal. - If the board becomes full with no winner, the game is a draw. - Once the game has a winner or is a draw, further move commands are ignored until a reset happens. - A reset clears the board and starts a new game with X. Return the final board state and the final game status. Use '.' to represent an empty cell in the returned board.

Constraints

0 <= len(commands) <= 100000
The board size is always 3 x 3
For a valid move command, row and col are intended to be in the range [0, 2]
Invalid moves into occupied cells or after the game is over must be ignored
Only successful moves switch the current player

Examples

Input: [(0,0), (1,0), (0,1), (1,1), (0,2)]

Expected Output: (['XXX', 'OO.', '...'], 'Winner: X')

Explanation: X fills the entire top row on the fifth move and wins.

Input: [(0,0), (0,1), (0,2), (1,1), (1,0), (1,2), (2,1), (2,0), (2,2)]

Expected Output: (['XOX', 'XOO', 'OXX'], 'Draw')

Explanation: All 9 cells are filled without any player completing a row, column, or diagonal.

Input: []

Expected Output: (['...', '...', '...'], 'In Progress')

Explanation: No commands means the initial empty board is unchanged.

Input: [(0,0), (0,0), (1,1), (0,1)]

Expected Output: (['XX.', '.O.', '...'], 'In Progress')

Explanation: The second command tries to use an occupied cell, so it is ignored and O keeps the turn for the next valid move.

Input: [(0,0), (1,0), (0,1), (1,1), (0,2), (2,2), "RESET", (2,2), (2,2)]

Expected Output: (['...', '...', '..X'], 'In Progress')

Explanation: After X wins, further moves are ignored until RESET. The reset clears the board, then X places one mark at the bottom-right.

Input: [(0,0), (1,1), (0,1), (1,0), (2,2), (1,2)]

Expected Output: (['XX.', 'OOO', '..X'], 'Winner: O')

Explanation: O completes the middle row on the last move.

Hints

Track four pieces of state: the board, the current player, the current status, and whether the game is over.
After placing a mark at (row, col), you only need to check that row, that column, and possibly the two diagonals.

Quick Overview

This question evaluates proficiency in interactive UI development and stateful game logic, including turn management, board state representation, win/draw detection, input validation, and reset flow.