## The challenge of solving valid Sudoku blocks

Determine if a 9×9 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 9
`3x3`

sub-boxes of the grid must contain the digits`1-9`

without repetition.

The Sudoku board could be partially filled, where empty cells are filled with the character `'.'`

.

**Example 1:**

Input:[ ["5","3",".",".","7",".",".",".","."], ["6",".",".","1","9","5",".",".","."], [".","9","8",".",".",".",".","6","."], ["8",".",".",".","6",".",".",".","3"], ["4",".",".","8",".","3",".",".","1"], ["7",".",".",".","2",".",".",".","6"], [".","6",".",".",".",".","2","8","."], [".",".",".","4","1","9",".",".","5"], [".",".",".",".","8",".",".","7","9"] ]Output:true

**Example 2:**

Input:[ ["8","3",".",".","7",".",".",".","."], ["6",".",".","1","9","5",".",".","."], [".","9","8",".",".",".",".","6","."], ["8",".",".",".","6",".",".",".","3"], ["4",".",".","8",".","3",".",".","1"], ["7",".",".",".","2",".",".",".","6"], [".","6",".",".",".",".","2","8","."], [".",".",".","4","1","9",".",".","5"], [".",".",".",".","8",".",".","7","9"] ]Output:falseExplanation:Same as Example 1, except with the5in the top left corner being modified to8. Since there are two 8's in the top left 3x3 sub-box, it is invalid.

**Note:**

- A Sudoku board (partially filled) could be valid but is not necessarily solvable.
- Only the filled cells need to be validated according to the mentioned rules.
- The given board contain only digits
`1-9`

and the character`'.'`

. - The given board size is always
`9x9`

.

## The solution written in Java

```
class Solution {
// Store our blocks
Set blockSet = new HashSet<>();
// Perform the check
public boolean isValidSudoku(char[][] board) {
// Row and Column variables to check against
Set rowset = new HashSet<>();
Set colset = new HashSet<>();
// Loop through the column
for(int i = 0; i<9; i++){
// Loop through the row
for(int j = 0; j<9; j++){
// False if exists in column
if(colset.contains(board[i][j]))
return false;
// False if exists in row
if(rowset.contains(board[j][i]))
return false;
// Add to column and row
if(board[i][j] != '.') colset.add(board[i][j]);
if(board[j][i] != '.') rowset.add(board[j][i]);
}
// Clear this run at the end
rowset.clear();
colset.clear();
}
// While we loop the board
int i = 0, j = 0;
while(i < 9 && j < 9) {
while(i < 9) {
// False if not valid
if(!isValid(board, i, j))
return false;
// Increment one section
i += 3;
// Clear the board
blockSet.clear();
}
// Next column/row
i = 0;
j += 3;
}
// Is valid!
return true;
}
// Check if a section is valid
private boolean isValid(char[][] board, int i, int j) {
// Loop the column
for(int x = i; x < i+3; x++) {
// Loop the row
for(int y = j; y < j+3; y++) {
// False if already in the set
if(blockSet.contains(board[x][y]))
return false;
// If still in a section, add it
if(board[x][y] != '.')
blockSet.add(board[x][y]);
}
}
// Is valid!
return true;
}
}
```

Code language: Java (java)