Design Tic-Tac-Toe

Requirements

• A 3x3 board; two players alternate placing their mark.
• Detect a win (3 in a row/column/diagonal), a draw (board full, no winner), and reject moves on an occupied square.
• Should generalize cleanly to an NxN board with a "K in a row to win" rule, without a rewrite.

Key classes to identify before you look at the reference

This problem is intentionally small — the interesting part isn't the classes themselves, it's whether your first design accidentally hard-codes "3x3" and "3 in a row" in a way that makes the NxN follow-up painful. Design for the general case from the start.