Verify Sudoku Board

Determine whether a 9×9 Sudoku board is valid.

A board is valid if:

• Each row contains no duplicate digits 1–9
• Each column contains no duplicates
• Each 3×3 subgrid contains no duplicates

Empty cells are represented by ".".

Input:
[
  ["5","3",".",".","7",".",".",".","."],
  ["6",".",".","1","9","5",".",".","."],
  ...
]

Output: true

export function isValidSudoku(board: string[][]): boolean {
    const rows = new Array(9).fill(null).map(() => new Set());
    const cols = new Array(9).fill(null).map(() => new Set());
    const boxes = new Array(9).fill(null).map(() => new Set());

  for (let r = 0; r < 9; r++) {
    for (let c = 0; c < 9; c++) {
      const val = board[r][c];
      if (val === ".") continue;

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

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

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

  return true;
}