任意模数FFT时记M为sqrt(mo)

将每个数a分为a/M,a%M后分别进行三次实数FFT

const LL mo=1e9+7;const LL M=sqrt(mo);const LDB pi=acos(-1);  LL cunt[511][511];  struct cp{      LDB r,i;  }a1[511][1024],a2[511][1024],tmp[1024];    inline cp operator +(cp a,cp b) {
   return((cp){a.r+b.r,a.i+b.i});};  inline cp operator -(cp a,cp b) {
   return((cp){a.r-b.r,a.i-b.i});};  inline cp operator *(cp a,cp b) {
   return((cp){a.r*b.r-a.i*b.i,a.i*b.r+a.r*b.i});};  void FFT(cp a[],int n,int fl){    for (int i=1,j=n/2;i
     
      >1);j&k;j^=k,k>>=1);      j^=k;    }        for (int i=2;i<=n;i<<=1){      cp w;w.r=cos(fl*2*pi/i);w.i=sin(fl*2*pi/i);      for (int j=0;j