36. Valid Sudoku

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

Determine if a 9 x 9 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 nine 3 x 3 sub-boxes of the grid must contain the digits 1-9 without repetition.

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.

Example 1:

Input: board = 
[["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: board = 
[["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.

Constraints:

• board.length == 9

• board[i].length == 9

• board[i][j] is a digit or '.'.

Solution

class Solution {
public:
    bool isValidSudoku(vector<vector<char>>& board) {
        bool check[9] = {false};
        for (const auto &row: board) {
            for (const auto e: row) {
                if (!isdigit(e)) continue;
                if (check[e - '1']) return false;
                check[e - '1'] = true;
            }
            memset(check, 0, 9);
        }
        
        for (int i = 0; i < 9; ++i) {
            for (int j = 0; j < 9; ++j) {
                const auto e = board[j][i];
                if (!isdigit(e)) continue;
                if (check[e - '1']) return false;
                check[e - '1'] = true;
            }
            memset(check, 0, 9);
        }
        
        for (int k = 0; k < 9; k += 3) {
            for (int i = 0; i < 9; ++i) {
                if (i % 3 == 0) memset(check, 0, 9);
                for (int j = k; j < k + 3; ++j) {
                    const auto e = board[i][j];
                    if (!isdigit(e)) continue;
                    if (check[e - '1']) return false;
                    check[e - '1'] = true;
                }
            }
        }
        return true;
    }
};