Build a TicTacToe class for two players, X and O, on a 3 by 3 board. One object is one game: it remembers the board and whose turn it is, lets the players take turns, and tells who has won.
The board: rows and columns are numbered 0, 1 and 2. Cell (0, 0) is the top-left corner, (0, 2) the top-right, (2, 0) the bottom-left and (2, 2) the bottom-right. A cell holds "X", "O" or "." when it is still empty.
Build:
- A new game. The class has two private properties, set up when the object is created:
board, a 3 by 3 array ofCharacterfilled with".", andplayer: Character = "X". Every row must be its own array. All nine cells are empty and X moves first. func move(_ row: Int, _ column: Int)plays one turn at the given row and column (always between 0 and 2, so you do not need to check the range).- If the cell is not empty, ignore the call: nothing changes and it is still the same player's turn.
- Otherwise put the current player's mark in the cell, then give the turn to the other player (X becomes O, O becomes X).
func winner() -> Characterreturns the winner as aCharacter:"X","O"or".".- A player wins by having the same mark in all three cells of a row, a column, or one of the two diagonals ((0,0), (1,1), (2,2) and (0,2), (1,1), (2,0)). Three empty cells in a line are not a win.
- If nobody has three in a row, return
".". That covers a game still in progress, an empty board and a draw.
You start with this code, which plays six games and prints the result of each one:
class Solution {
func mainFunction() {
var game = TicTacToe()
game.move(0, 0)
game.move(1, 0)
game.move(0, 1)
game.move(1, 1)
game.move(0, 2)
print("\(game.winner()) wins!")
game = TicTacToe()
game.move(0, 0)
game.move(0, 1)
game.move(0, 0)
game.move(1, 0)
game.move(1, 1)
game.move(2, 2)
game.move(2, 1)
print("\(game.winner()) wins!")
game = TicTacToe()
game.move(0, 0)
game.move(0, 1)
game.move(1, 1)
game.move(0, 2)
game.move(2, 2)
print("\(game.winner()) wins!")
game = TicTacToe()
game.move(0, 2)
game.move(0, 0)
game.move(1, 1)
game.move(0, 1)
game.move(2, 0)
print("\(game.winner()) wins!")
game = TicTacToe()
game.move(0, 0)
game.move(0, 1)
game.move(0, 2)
game.move(1, 1)
game.move(1, 0)
game.move(1, 2)
game.move(2, 1)
game.move(2, 0)
game.move(2, 2)
print("No winner: \(game.winner())")
game = TicTacToe()
print("No winner: \(game.winner())")
}
}
Games played: X fills the top row; then a game where X tries to play on a cell that is already taken (ignored, and still X's turn) and O wins the middle column; X on the main diagonal; X on the other diagonal; a full board with no three in a row; and a brand-new game.
Expected output:
X wins! O wins! X wins! X wins! No winner: . No winner: .
There are eight winning lines: three rows, three columns and two diagonals. A loop over
i from 0 to 2 can check row i and column i together; the two diagonals come after the loop. Skip a line whose first cell is empty.
Check for a taken cell first and leave the move method before touching the board or the turn. Only a successful move passes the turn to the other player.
class TicTacToe {
private var board = Array(repeating: Array(repeating: Character("."), count: 3), count: 3)
private var player: Character = "X"
func move(_ row: Int, _ column: Int) {
if board[row][column] != "." { return }
board[row][column] = player
player = player == "X" ? "O" : "X"
}
func winner() -> Character {
for index in 0..<3 {
if board[index][0] != "." && board[index][0] == board[index][1] && board[index][1] == board[index][2] { return board[index][0] }
if board[0][index] != "." && board[0][index] == board[1][index] && board[1][index] == board[2][index] { return board[0][index] }
}
if board[0][0] != "." && board[0][0] == board[1][1] && board[1][1] == board[2][2] { return board[0][0] }
if board[0][2] != "." && board[0][2] == board[1][1] && board[1][1] == board[2][0] { return board[0][2] }
return "."
}
}
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