slow-sudoku.cc

class Solution {
public:
    void solveSudoku(vector<vector<char>>& board) {
        for (int row = 0; row < 9; row ++) {
            auto rowi = &board[row];
            for (int col = 0; col < 9; col ++) {
                if ((*rowi)[col] == '.') {
                    for (char a = '1'; a <= '9'; ++ a) {
                        if (can(board, col, row, a)) {
                            (*rowi)[col] = a;
                            solveSudoku(board);
                            if (isSolution(board)) {
                                return;
                            }
                        }
                    }
                    (*rowi)[col] = '.';
                    return;
                }
            }
        }
    }
    
    bool can(vector<vector<char>>& board, int col, int row, char val) {
        auto v = &board[row];
        for (auto i = v->begin(); i != v->end(); ++ i) {
            if (*i == val) {
                return false;
            }
        }
        
        for (int i = 0; i < 9; i ++) {
            if (board[i][col] == val) {
                return false;
            }
        }
        
        const int sj = row / 3;
        const int si = col / 3;
        for (int j = sj * 3; j < sj * 3 + 3; j ++) {
            for (int i = si * 3; i < si * 3 + 3; i ++) {
                if (board[j][i] == val) {
                    return false;
                }
            }
        }
        
        return true;
    }
    
    bool isSolution(vector<vector<char>>& board) {
        for (auto row = board.begin(); row != board.end(); ++ row) {
            for (auto col = row->begin(); col != row->end(); ++ col) {
                if (*col == '.') {
                    return false;
                }
            }
        }
        return true;
    }
};