## 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; } }