Object-Oriented Design Interview (Neetcode.io)
19 ๐Ÿ‘€
Harry Potter

Harry Potter

Sep 14, 2023

Object-Oriented Design Interview (Neetcode.io)

A better way to prepare for coding interviews.

Design Connect Four


Connect Four is a popular game played on a 7x6 grid. Two players take turns dropping colored discs into the grid. The first player to get four discs in a row (vertically, horizontally or diagonally) wins.

Connect Four


Some possible questions to ask:

  • What are the rules of the game?
  • What size is the grid?
  • How many players are there? Player vs Computer? Player vs Player?
  • Are we keeping track of the score?


  • The game will be played by only two players, player vs player
  • The game board should be of variable dimensions
  • The target is to connect N discs in a row (vertically, horizontally or diagonally)
    • N is a variable (e.g. connect 4, 5, 6, etc)
  • There should be a score tracking system
    • After a player reaches the target score, they are the winner



  • We will need a Grid class to maintain the state of the 2-D board
    • The board cell can be empty, yellow (occupied by Player 1) or red (occupied by Player 2)
    • The grid will also be responsible for checking for a win condition
  • We can have a Player class to represent the player's piece color
    • This isn't super important, but encapsulating information is generally a good practice
  • The Game class will be composed of the Grid and Players
    • The Game class will be responsible for the game loop and keeping track of the score


We will use an enum to represent the GridPosition.

import enum

class GridPosition(enum.Enum):
    EMPTY = 0
    YELLOW = 1
    RED = 2

The Grid will maintain the state of the board and all of the pieces. It will also check for a win condition. Perhaps it would be more appropriate to name the checkWin method to checkNConnected, since the Grid itself shouldn't need to know what the rules of the game are.

class Grid:
    def __init__(self, rows, columns):
        self._rows = rows
        self._columns = columns
        self._grid = None

    def initGrid(self):
        self._grid = [[GridPosition.EMPTY for _ in range(self._columns)] for _ in range(self._rows)]

    def getGrid(self):
        return self._grid

    def getColumnCount(self):
        return self._columns

    def placePiece(self, column, piece):
        if column < 0 or column >= self._columns:
            raise ValueError('Invalid column')
        if piece == GridPosition.EMPTY:
            raise ValueError('Invalid piece')
        for row in range(self._rows-1, -1, -1):
            if self._grid[row][column] == GridPosition.EMPTY:
                self._grid[row][column] = piece
                return row

    def checkWin(self, connectN, row, col, piece):
        count = 0
        # Check horizontal
        for c in range(self._columns):
            if self._grid[row][c] == piece:
                count += 1
                count = 0
            if count == connectN:
                return True

        # Check vertical
        count = 0
        for r in range(self._rows):
            if self._grid[r][col] == piece:
                count += 1
                count = 0
            if count == connectN:
                return True

        # Check diagonal
        count = 0
        for r in range(self._rows):
            c = row + col - r
            if c >= 0 and c < self._columns and self._grid[r][c] == piece:
                count += 1
                count = 0
            if count == connectN:
                return True

        # Check anti-diagonal
        count = 0
        for r in range(self._rows):
            c = col - row + r
            if c >= 0 and c < self._columns and self._grid[r][c] == piece:
                count += 1
                count = 0
            if count == connectN:
                return True

        return False

Player is only mean to encapsulate the player's information, more importantly the player's piece color.

class Player:
    def __init__(self, name, pieceColor):
        self._name = name
        self._pieceColor = pieceColor

    def getName(self):
        return self._name

    def getPieceColor(self):
        return self._pieceColor

The Game class will be used to play the game. It will keep track of the players, the score, and the grid. It will also be responsible for the game loop. The game parameters passed in via the constructor give us flexibility to play the game with slightly different rules and dimensions.
While we could instantiate the board within the Game class, it's preferred to pass it in via the constructor. This means the Game class does not need to know how to instantiate the board.
Even though we are only playing with two players, we can still use a list to store the players. This is not necessary, but it's easy enough and gives us flexibility to add more players in the future.

class Game:
    def __init__(self, grid, connectN, targetScore):
        self._grid = grid
        self._connectN = connectN
        self._targetScore = targetScore

        self._players = [
            Player('Player 1', GridPosition.YELLOW),
            Player('Player 2', GridPosition.RED)

        self._score = {}
        for player in self._players:
            self._score[player.getName()] = 0

    def printBoard(self):
        grid = self._grid.getGrid()
        for i in range(len(grid)):
            row = ''
            for piece in grid[i]:
                if piece == GridPosition.EMPTY:
                    row += '0 '
                elif piece == GridPosition.YELLOW:
                    row += 'Y '
                elif piece == GridPosition.RED:
                    row += 'R '

    def playMove(self, player):
        print(f"{player.getName()}'s turn")
        colCnt = self._grid.getColumnCount()
        moveColumn = int(input(f"Enter column between {0} and {colCnt - 1} to add piece: "))
        moveRow = self._grid.placePiece(moveColumn, player.getPieceColor())
        return (moveRow, moveColumn)

    def playRound(self):
        while True:
            for player in self._players:
                row, col = self.playMove(player)
                pieceColor = player.getPieceColor()
                if self._grid.checkWin(self._connectN, row, col, pieceColor):
                    self._score[player.getName()] += 1
                    return player

    def play(self):
        maxScore = 0
        winner = None
        while maxScore < self._targetScore:
            winner = self.playRound()
            print(f"{winner.getName()} won the round")
            maxScore = max(self._score[winner.getName()], maxScore)

            self._grid.initGrid() # reset grid
        print(f"{winner.getName()} won the game")

Finally, we can create the grid, set the game parameters, and play the game.

grid = Grid(6, 7)
game = Game(grid, 4, 2)

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Harry Potter

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