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Implement Connect Four game | Airbnb Coding Question

Quick Overview

This question evaluates implementation skills for game state management, move handling, and efficient winner-detection algorithms, testing competence in data structures, algorithmic complexity analysis, and correctness under edge cases.

Implement Connect Four game

Company: Airbnb

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: medium

Interview Round: Technical Screen

##### Question Design and implement the Connect Four game board, player moves, and winner-detection logic. Discuss time/space complexity and possible follow-ups such as AI opponent or scalable multiplayer service.

Quick Answer: This question evaluates implementation skills for game state management, move handling, and efficient winner-detection algorithms, testing competence in data structures, algorithmic complexity analysis, and correctness under edge cases.

Implement the core logic of a standard Connect Four game. The board has 6 rows and 7 columns. Players alternate turns, with Red ('R') moving first and Yellow ('Y') moving second. Each move is a column index, and the piece falls to the lowest available cell in that column. Process the moves in order and stop immediately if a player wins. A player wins if their newly placed piece creates 4 consecutive pieces horizontally, vertically, or diagonally. Return: - 'R' if Red wins - 'Y' if Yellow wins - 'Draw' if the board becomes full with no winner - 'Pending' if all moves are processed and no winner exists but the board is not full - 'Invalid' if a move uses a column outside 0 to 6, or tries to place a piece into a full column before the game has ended

Constraints

The board size is fixed at 6 rows by 7 columns.
0 <= len(moves) <= 100
A valid column index is an integer from 0 to 6.

Examples

Input: ([],)

Expected Output: 'Pending'

Explanation: No moves have been played, so nobody has won and the board is not full.

Input: ([3, 3, 2, 2, 1, 1, 0],)

Expected Output: 'R'

Explanation: Red forms a horizontal line on the bottom row across columns 0, 1, 2, and 3.

Input: ([0, 1, 0, 1, 2, 1, 3, 1],)

Expected Output: 'Y'

Explanation: Yellow stacks 4 pieces vertically in column 1.

Input: ([0, 1, 1, 2, 4, 2, 2, 3, 4, 3, 5, 3, 3],)

Expected Output: 'R'

Explanation: Red creates a diagonal from bottom-left to top-right.

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

Expected Output: 'Invalid'

Explanation: A Connect Four column holds only 6 pieces. The 7th move in the same column is invalid.

Hints

Keep track of the next open row in each column so you can place a piece in O(1) time.
After placing a piece, only check lines that pass through that new piece: horizontal, vertical, and the two diagonals.

Last updated: Jun 2, 2026

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