You're building a Tic-Tac-Toe game. The board is a 3x3 grid where players place X or O. You need to efficiently check if a cell is empty, place a mark, and detect winners.

Code

enum Mark { EMPTY, X, O }

class Board {
  private grid: Mark[][] = [
    [Mark.EMPTY, Mark.EMPTY, Mark.EMPTY],
    [Mark.EMPTY, Mark.EMPTY, Mark.EMPTY],
    [Mark.EMPTY, Mark.EMPTY, Mark.EMPTY],
  ]

  // O(1) win detection tracking
  private rowCounts: number[][] = [[0,0],[0,0],[0,0]]  // [row][X=0,O=1]
  private colCounts: number[][] = [[0,0],[0,0],[0,0]]
  private diagCounts: number[] = [0, 0]  // main diag
  private antiDiagCounts: number[] = [0, 0]
  private moveCount: number = 0

  placeMark(row: number, col: number, mark: Mark): boolean {
    if (this.grid[row][col] !== Mark.EMPTY) return false
    this.grid[row][col] = mark
    this.moveCount++
    // Update counts for O(1) win check
    return true
  }
}

Question

What's the best way to represent the 3x3 board?

0% completed

🔍Search

📚LLD Introduction

🎯OOP Fundamentals

🔗Class Relationships

💡Design Principles

⭐SOLID Principles

Single Responsibility Principle

Open/Closed Principle

Liskov Substitution Principle

Interface Segregation Principle

Dependency Inversion Principle

📊UML Diagrams

🎨Additional Patterns

Template Method Pattern

Proxy Pattern

💼LLD Interview Tips

LLD Interview Approach

Common LLD Mistakes

Evaluation Criteria

Back to LLD

9/20

LLD|❓ LLD Interview Questions| 25 min

Design Tic Tac Toe

Intermediate25 min read+10 XP

TL;DR — Design Tic Tac Toe

Learn Design Tic Tac Toe — a key concept for LLD interviews.

Interactive — Try it yourself!

❌Design Tic-Tac-Toeeasy

Step 1/5

0 pts

Board Design

Efficient State Tracking

You're building a Tic-Tac-Toe game. The board is a 3x3 grid where players place X or O. You need to efficiently check if a cell is empty, place a mark, and detect winners.

Code

enum Mark { EMPTY, X, O }

class Board {
  private grid: Mark[][] = [
    [Mark.EMPTY, Mark.EMPTY, Mark.EMPTY],
    [Mark.EMPTY, Mark.EMPTY, Mark.EMPTY],
    [Mark.EMPTY, Mark.EMPTY, Mark.EMPTY],
  ]

  // O(1) win detection tracking
  private rowCounts: number[][] = [[0,0],[0,0],[0,0]]  // [row][X=0,O=1]
  private colCounts: number[][] = [[0,0],[0,0],[0,0]]
  private diagCounts: number[] = [0, 0]  // main diag
  private antiDiagCounts: number[] = [0, 0]
  private moveCount: number = 0

  placeMark(row: number, col: number, mark: Mark): boolean {
    if (this.grid[row][col] !== Mark.EMPTY) return false
    this.grid[row][col] = mark
    this.moveCount++
    // Update counts for O(1) win check
    return true
  }
}

Question

What's the best way to represent the 3x3 board?