迭代FFT

int reverse(int x, int len){
    int t = 0;
    for (int i = 1; i < len; i <<= 1){
        t <<= 1;
        if (x & i) t |= 1;
    }
    return t;
}

Complex A[200010];
void FFT(Complex *a, int n, int DFT){
    for (int i = 0; i < n; ++i) A[reverse(i, n)] = a[i];
    for (int i = 2; i <= n; i <<= 1){
        Complex wn = Complex(cos(2 * pi / i), DFT * sin(2 * pi / i));
        for (int j = 0; j < n; j += i){
            Complex w = Complex(1, 0);
            for (int k = 0; k < (i >> 1); ++k){
                Complex x = A[j + k];
                Complex y = w * A[j + k + (i >> 1)];
                A[j + k] = x + y;
                A[j + k + (i >> 1)] = x - y;
                w = w * wn;
            }
        }
    }
    if (DFT == -1) for (int i = 0; i < n; ++i) A[i] = A[i] / n;
    for (int i = 0; i < n; ++i) a[i] = A[i];
}