1 bool row[9][9]; 2 bool col[9][9]; 3 bool grid[9][9]; 4 bool mark[9][9]; 5  6 bool solve(vector<vector<char> > &board, int r, int c) { 7   if (r == 9) 8     return true; 9   if (mark[r][c])10     return solve(board, r + (c + 1) / 9, (c + 1) % 9);11   for (int i = 0; i < 9; i++) {12     if (!row[r][i] && !col[c][i] && !grid[(r / 3) * 3 + c / 3][i]) {13       row[r][i] = col[c][i] = grid[(r / 3) * 3 + c / 3][i] = true;14       board[r][c] = i + ‘1‘;15       if (solve(board, r + (c + 1) / 9, (c + 1) % 9))16         return true;17       board[r][c] = ‘.‘;18       row[r][i] = col[c][i] = grid[(r / 3) * 3 + c / 3][i] = false;19     }20   }21 22   return false;23 }24 25 void solveSudoku(vector<vector<char> > &board) {26   memset(row, 9 * 9 * sizeof(bool), false);27   memset(col, 9 * 9 * sizeof(bool), false);28   memset(grid, 9 * 9 * sizeof(bool), false);29   memset(mark, 9 * 9 * sizeof(bool), false);30 31   for (int r = 0; r < 9; r++)32     for (int c = 0; c < 9; c++)33       if (board[r][c] != ‘.‘) {34         row[r][board[r][c] - ‘1‘]35           = col[c][board[r][c] - ‘1‘]36           = grid[(r / 3) * 3 + c / 3][board[r][c] - ‘1‘]37           = mark[r][c]38           = true;39       }40 41   solve(board, 0, 0);42 }