LeetCode Challenge: Validate a Sudoku Board in JavaScript

LeetCode Challenge: Solve Valid Sudoku in JavaScript

A Step-by-Step Guide to Problem #36

🚀 Problem Description

Given a 9x9 Sudoku board, your task is to validate its current configuration. The board is valid if it adheres to the following rules:

• Row Validation: Each row must contain unique digits from 1-9.
• Column Validation: Each column must contain unique digits from 1-9.
• 3x3 Sub-Box Validation: Each of the nine 3x3 grids must also contain unique digits from 1-9.

Additional Notes:

• Empty cells are represented by "." and are ignored during validation.
• You do not need to solve the board; only validate the current configuration.

💡 Examples

Example 1: Valid Board

const board = [
  ["5", "3", ".", ".", "7", ".", ".", ".", "."],
  ["6", ".", ".", "1", "9", "5", ".", ".", "."],
  [".", "9", "8", ".", ".", ".", ".", "6", "."],
  ["8", ".", ".", ".", "6", ".", ".", ".", "3"],
  ["4", ".", ".", "8", ".", "3", ".", ".", "1"],
  ["7", ".", ".", ".", "2", ".", ".", ".", "6"],
  [".", "6", ".", ".", ".", ".", "2", "8", "."],
  [".", ".", ".", "4", "1", "9", ".", ".", "5"],
  [".", ".", ".", ".", "8", ".", ".", "7", "9"],
];
console.log(isValidSudoku(board)); // Output: true

Example 2: Invalid Board

const board = [
  ["8", "3", ".", ".", "7", ".", ".", ".", "."],
  ["6", ".", ".", "1", "9", "5", ".", ".", "."],
  [".", "9", "8", ".", ".", ".", ".", "6", "."],
  ["8", ".", ".", ".", "6", ".", ".", ".", "3"],
  ["4", ".", ".", "8", ".", "3", ".", ".", "1"],
  ["7", ".", ".", ".", "2", ".", ".", ".", "6"],
  [".", "6", ".", ".", ".", ".", "2", "8", "."],
  [".", ".", ".", "4", "1", "9", ".", ".", "5"],
  [".", ".", ".", ".", "8", ".", ".", "7", "9"],
];
console.log(isValidSudoku(board)); // Output: false

🏆 JavaScript Solution

To validate the board, we use three key checks:

• Rows: Ensure each row has unique digits.
• Columns: Ensure each column has unique digits.
• Sub-Boxes: Validate all nine 3x3 grids for unique digits.

Implementation

function isValidSudoku(board) {
  const rows = Array.from({ length: 9 }, () => new Set());
  const cols = Array.from({ length: 9 }, () => new Set());
  const boxes = Array.from({ length: 9 }, () => new Set());

  for (let r = 0; r < 9; r++) {
    for (let c = 0; c < 9; c++) {
      const value = board[r][c];

      if (value === ".") continue;

      const boxIndex = Math.floor(r / 3) * 3 + Math.floor(c / 3);

      if (rows[r].has(value) || cols[c].has(value) || boxes[boxIndex].has(value)) {
        return false;
      }

      rows[r].add(value);
      cols[c].add(value);
      boxes[boxIndex].add(value);
    }
  }

  return true;
}

🔍 How It Works

1. Initialization: Create three arrays of Set objects for tracking numbers in rows, columns, and sub-boxes.
2. Iterate through the board:
   • Skip empty cells (".").
   • Compute the box index dynamically: Math.floor(r / 3) * 3 + Math.floor(c / 3).
   • Check for duplicates in the corresponding row, column, or sub-box. If found, return false.
   • Add the value to the respective row, column, and box sets.
3. Return Result: If all checks pass, return true.

📊 Complexity Analysis

Time Complexity: O(81) → O(1) (since the board size is fixed).

Space Complexity: O(27) → O(1) (27 sets for rows, columns, and sub-boxes).

✨ Pro Tips for Coding Interviews

• Ask clarifying questions about constraints and input format.
• Explain your approach using sets for efficient validation.
• Discuss edge cases such as empty boards or nearly full invalid boards.
• Keep your code clean and readable with descriptive variable names.

Keywords: Validate Sudoku JavaScript, LeetCode Solutions, JavaScript Algorithms, Sudoku Validation, Coding Interview Guide