36. Valid Sudoku

之前没有写过这么简洁的方法：

class Solution {
public:
    bool isValidSudoku(vector<vector<char>>& board) {
        int used1[9][9] = {0}, used2[9][9] = {0}, used3[9][9] = {0};
        for (int i = 0; i < 9; ++i)
            for (int j = 0; j < 9; ++j) {
                if (board[i][j] == '.')
                    continue;
                int num = board[i][j] - '0' - 1;
                int k = i / 3 * 3 + j / 3;
                if (used1[i][num] || used2[j][num] || used3[k][num])
                    return false;
                used1[i][num] = used2[j][num] = used3[k][num] = 1;
            }
        return true;
    }
};
//用三个额外的二维数组，分别检测横，纵，块。