void medianFilter0(const size_t& radius) {
        std::vector<grayvalue_t> temp(_pixels.size());
        size_t i, j;
        for (i = 0; i < R(); i++){
            for (j = 0; j < C(); j++){
                grayvalue_t ans = calc_neighbors(radius,i,j);
                temp[i*C() + j] = ans;
            }
        }
        for (i = 0; i < R(); i++){
            for (j = 0; j < C(); j++){
                _pixels[i*C() + j]=temp[i*C() + j];
            }
        }
    } // medianFilter0
    grayvalue_t calc_neighbors(const size_t& radius, size_t i, size_t j){
        std::vector<grayvalue_t> tempvec((2 * radius + 1)*(2 * radius + 1);
        size_t ii,jj,neighbors_num = 0;
        for (ii = i - radius; ii <= i + radius; i++){
            for (jj = j - radius; jj <= j + radius; j++){
                if (isInside(ii,jj){
                    tempvec[neighbors_num++] = getPixel(ii, jj);
                }
            }
        }
        std::sort(tempvec.begin(), tempvec.begin() + neighbors_num);
        return tempvec[(neighbors_num - 1) / 2];
    }
    bool isInside(size_t i, size_t j){
        return i >= 0 && j >= 0 && i < R() && j < C();
    }