迭代FFT

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

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