void setPixel(const size_t& r, const size_t& c, grayvalue_t t){
        assert(r<R() && c<C());
        _pixels[r*C() + c]=t;
    }
    size_t getPixelTemp(const size_t& r, const size_t& c){
        assert(r<R() && c<C());
        return r*C() + c;
    }
    void medianFilter0(const size_t& radius, size_t flag) {
        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,flag);
                temp[getPixelTemp(i,j)] = ans;
            }
        }
        for (i = 0; i < R(); i++){
            for (j = 0; j < C(); j++){
                setPixel(i, j, temp[getPixelTemp(i, j)]);
            }
        }
    } // medianFilter0
    grayvalue_t calc_neighbors(const size_t& radius, size_t i, size_t j,size_t flag){
        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; ii++){
            for (jj = j - radius; jj <= j + radius; jj++){
                if (isInside(ii,jj){
                    tempvec[neighbors_num++] = getPixel(ii, jj);
                }
            }
        }
        if (flag == 0)std::sort(tempvec.begin(), tempvec.begin() + neighbors_num);
        else std::nth_element(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();
    }
    void medianFilter1(const size_t& radius) {
        medianFilter0(radius, 1);
    } // medianFilter