On-chain tic-tac-toe

• Two addresses: playerX (deployer) and playerO (first to call join)
• 3×3 board as uint8[9]: 0=empty, 1=X, 2=O
• turn counter: even = X's turn, odd = O's turn
• place(uint8 pos) — only the current player can call, cell must be empty
• Winner check after each move: 8 winning lines

solidity
contract TicTacToe {
    address public playerX;
    address public playerO;
    uint8[9] public board;
    uint256 public turn;
    uint8 public winner;   // 0=none, 1=X, 2=O, 3=draw

    event Move(address indexed player, uint8 position, uint256 turn);
    event GameOver(uint8 winner);

    constructor() { playerX = msg.sender; }

    function join() external {
        require(playerO == address(0), "game full");
        require(msg.sender != playerX, "can't play yourself");
        playerO = msg.sender;
    }

    function place(uint8 pos) external {
        require(winner == 0, "game over");
        require(pos < 9 && board[pos] == 0, "bad position");
        address expected = turn % 2 == 0 ? playerX : playerO;
        require(msg.sender == expected, "not your turn");
        board[pos] = turn % 2 == 0 ? 1 : 2;
        emit Move(msg.sender, pos, turn);
        turn++;
        // check winner…
    }
}

Eight lines: 3 rows, 3 columns, 2 diagonals. Each line is an array of 3 positions. If all three have the same non-zero value → that player wins.

Write the place function, and implement checkWin() which sets the winner state if there's a 3-in-a-row.