Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules: Each row must contain the digits 1-9 without repetition. Each column must contain the digits 1-9 without repetition. Each of the

isValidSudoku.js

/**
 * Validates a partially filled 9x9 Sudoku board.
 * Only filled cells are checked for rule violations.
 *
 * @param {character[][]} board - 2D array representing the Sudoku board
 * @return {boolean} - true if the board is valid, false otherwise
 */
var isValidSudoku = function(board) {
    // Set to track seen digits in rows, columns, and sub-boxes
    const seen = new Set();

    // Iterate over each cell in the 9x9 grid
    for (let row = 0; row < 9; row++) {
        for (let col = 0; col < 9; col++) {
            const value = board[row][col];

            // Skip empty cells
            if (value === '.') continue;

            // Encode the presence of the digit in row, column, and sub-box
            const rowKey = `row-${row}-${value}`;
            const colKey = `col-${col}-${value}`;
            const boxKey = `box-${Math.floor(row / 3)}-${Math.floor(col / 3)}-${value}`;

            // If any of these keys already exist, it's a duplicate → invalid
            if (seen.has(rowKey) || seen.has(colKey) || seen.has(boxKey)) {
                return false;
            }

            // Otherwise, record the digit's presence
            seen.add(rowKey);
            seen.add(colKey);
            seen.add(boxKey);
        }
    }

    // No violations found
    return true;
};