Design a Tic-Tac-Toe / Chess Game

Board games are a favorite LLD prompt because they scale smoothly with the candidate's level. Tic-tac-toe is a 10-minute warm-up that tests basic modeling and win detection; chess is the senior version, where polymorphic piece movement, move validation, undo, and a game state machine all come into play. A strong candidate frames the simple version first, then upgrades the design without rewriting it.

What the interviewer is testing

• Do you separate the board (a grid of cells) from the rules (what constitutes a legal move) from the game loop (turns, win/draw detection)?
• For chess: do you make movement polymorphic per piece — each piece validates its own moves — instead of one monstrous isValidMove() with a switch on piece type?
• Can you support undo/redo cleanly? This is the tell that you know the Command pattern.
• Do you model the game's lifecycle (in-progress → check → checkmate / stalemate) as an explicit state machine rather than scattered boolean flags?

How to frame it

Start with tic-tac-toe to lock the shared vocabulary: Board, Cell, Player, Move, and a Game that alternates turns and checks rows/cols/diagonals for a win. State that this is the skeleton — then say "chess keeps the same shape but specializes three things: pieces, move validation, and game state."

For chess, the key decisions:

• Factory creates pieces for the initial setup, keeping construction in one place.
• Strategy / polymorphism — each Piece subtype implements canMove(board, from, to). The Rook knows about straight lines; the Knight knows its L-shape. No central type-switch.
• Command — each Move is a command with execute() and undo(), capturing the captured piece so the board can be restored. This is what makes undo/redo a stack of commands.
• Observer — the UI (or a logger / network peer) subscribes to board changes and re-renders.
• State — a GameState enum/object drives whether moves are accepted and detects check, checkmate, and stalemate.

The worked solution below uses chess as the running example and follows the full 9-section structure.