export function isValidSudoku(board: string[][]): boolean {
  const N = board.length;
  const rows = new Set();
  const colls = new Set();
  const cells = new Set();

  for (let i = 0; i < N; i++) {
    for (let j = 0; j < N; j++) {
      const val = board[i][j];
      if (val === ".") continue;
      const cellIndex = Math.floor(i / 3) * 3 + Math.floor(j / 3);
      const rowKey = i + "_" + val;
      const collKey = j + "_" + val;
      const cellKey = cellIndex + "_" + val;

      if (rows.has(rowKey) || colls.has(collKey) || cells.has(cellKey)) return false;

      rows.add(rowKey);
      colls.add(collKey);
      cells.add(cellKey);
    }
  }

  return true
};