Hướng dẫn check valid sudoku board
===

### Video
https://youtu.be/4RsKPnqf0ag

### Code mẫu

function sudoku(grid) {
  for (let k = 1; k <= 9; k++) {
    for (let i = 0; i < 9; i++) {
      if (!checkRow(grid, i, k)) {
          console.log(`false at row ${i} with k = ${k}`)
          return false;
      };
    }
    
    for (let i = 0; i < 9; i++) {
      if (!checkCol(grid, i, k)) {
        console.log(`false at col ${i} with k = ${k}`)
        return false;
      };
    }
    
    for (let i = 0; i < 3; i++) {
      for (let j = 0; j < 3; j++) {
        if (!checkBox(grid, i, j, k)) {
            console.log(`false at box ${i}, ${j} with k = ${k}`)
            return false;
        };
      }
    }
  }
  return true;
}

### TODO
- Implement 3 hàm `checkRow`, `checkCol`, `checkBox`