# 36. Valid Sudoku (Medium)

https://leetcode.com/problems/valid-sudoku/

Determine if a 9x9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

1. Each row must contain the digits `1-9` without repetition.
2. Each column must contain the digits `1-9` without repetition.
3. Each of the 9 `3x3` sub-boxes of the grid must contain the digits `1-9` without repetition.

A partially filled sudoku which is valid.

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: false
Explanation: Same as Example 1, except with the 5 in the top left corner being
modified to 8. 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`.

## Solutions

``````class Solution {
public boolean isValidSudoku(char[][] board) {
int row = board.length;
int col = board[0].length;

// check rows
for (int i = 0; i < row; i++) {
Set<Character> set = new HashSet<>();
for (int j = 0; j < col; j++) {
if (board[i][j] != '.' && !set.add(board[i][j])) {
return false;
}
}
}

// check columns
for (int i = 0; i < col; i++) {
Set<Character> set = new HashSet<>();
for (int j = 0; j < row; j++) {
if (board[j][i] != '.' && !set.add(board[j][i])) {
return false;
}
}
}

// check sub squares
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
Set<Character> set = new HashSet<>();
for (int x = i * 3; x < i * 3 + 3; x++) {
for (int y = j * 3; y < j * 3 + 3; y++) {
if (board[x][y] != '.' && !set.add(board[x][y])) {
return false;
}
}
}
}
}

return true;
}
}
``````