refactored_code_sudoku.dart

// Challenge 1
// Write Test cases for given challenges

// Challenge 2
// Sudoku Validation
// Write a sudoku validator. This function should return true if the 2-D array represents a valid sudoku and false otherwise. To be a valid sudoku:
//
// - Each row must have the digits from 1 to 9 exactly once.
// - Each column must have the digits from 1 to 9 exactly once.
// - Each 3x3 box must have the digits from 1 to 9 exactly once.
// Examples
//  sudokuValidator([
//  [ 1, 5, 2, 4, 8, 9, 3, 7, 6 ],
//  [ 7, 3, 9, 2, 5, 6, 8, 4, 1 ],
//  [ 4, 6, 8, 3, 7, 1, 2, 9, 5 ],
//  [ 3, 8, 7, 1, 2, 4, 6, 5, 9 ],
//  [ 5, 9, 1, 7, 6, 3, 4, 2, 8 ],
//  [ 2, 4, 6, 8, 9, 5, 7, 1, 3 ],
//  [ 9, 1, 4, 6, 3, 7, 5, 8, 2 ],
//  [ 6, 2, 5, 9, 4, 8, 1, 3, 7 ],
//  [ 8, 7, 3, 5, 1, 2, 9, 6, 4 ]
//  ]) ➞ true
bool sudokuValidator(List<List> board) {
  return isSudokuBoardOfRightSize(board) &&
      doCellsContainValidNumber(board) &&
      checkRowCol(board) &&
      checkBoxes(board);
}

//lets check if the board is valid or not first
// that is, size of the board should be multiple of 3
bool isSudokuBoardOfRightSize(List<List> board) {
  for (List row in board) {
    if (row.length % 3 != 0 && board.length % 3 != 0) {
      return false;
    }
  }
  print('right size');
  return true;
}

//now lets check if the sudokuBoard contains valid numbers as per its size
bool doCellsContainValidNumber(List<List> board) {
  int length =
      board.length; //length of board or number of rows in board and their size
  //so all numbers should be form 0 to length
  for (int row = 0; row < length; row++) {
    for (int col = 0; col < length; col++) {
      if (board[row][col] > length ||
          board[row][col] < 0 ||
          board[row][col] == 0) {
        return false;
      }
    }
  }
  print('valid numbers');
  return true;
}

//now lets check each row and also each column for duplicates
bool checkRowCol(List<List> board) {
  int length = board.length;
  //checking rows first
  for (int row = 0; row < length; row++) {
    if (board[row].toSet().length != length) {
      return false;
    }
  }
  //checking columns
  List colElementsList = [];
  for (int row = 0; row < length; row++) {
    for (int col = 0; col < length; col++) {
      colElementsList.add(board[col][row]);
    }
    if (colElementsList.toSet().length != length) {
      return false;
    }
  }
  print('row & col uniqe');
  return true;
}

//now lets check if all the subBoards of size 3
//don't have any repeating numbers
bool checkBoxes(List<List> board) {
  for (int row = 0; row < board.length; row = row + 3) {
    for (int col = 0; col < board.length; col = col + 3) {
//      print('row $row col $col');
      if (!isBoardBoxValid(board, row, col)) {
        return false;
      }
    }
  }
  print('all boxes unique');
  return true;
}

bool isBoardBoxValid(List<List> board, int rowPos, int colPos) {
  List elements = [];

  for (int row = rowPos; row < rowPos + 3; row++) {
    for (int col = colPos; col < colPos + 3; col++) {
      elements.add(board[row][col]);
    }
  }
//  print(elements); // debug print statement
  if (elements.toSet().length != board.length) {
    return false;
  }
  return true;
}