求助

#include<bits/stdc++.h>
using namespace std;
const int N=1e7+1;
const double Pi=acos(-1.0);
struct node{
	double x,y;
	node(double xx=0,double yy=0){
		x=xx,y=yy;
	}
}a[N],b[N];
int kkk[N];
node operator+(node a,node b){
	return node(a.x+b.x,a.y+b.y);
}
node operator-(node a,node b){
	return node(a.x-b.x,a.y-b.y);
}
node operator*(node a,node b){
	return node(a.x*b.x-a.y*b.y,a.x*b.y+a.y*b.x);
}
int n,m,l,r[N],limit=1;
void FFT(node *A,int type){
	for(int i=0;i<limit;++i){
		if(i<r[i])swap(A[i],A[r[i]]);
	}
	for(int mid=1;mid<limit;mid<<=1){
		node Wn(cos(Pi/mid),type*sin(Pi/mid));
		for(int r=mid<<1,j=0;j<limit;j+=r){
			node w(1,0);
			for(int k=0;k<mid;k++,w=w*Wn){
				node x=A[j+k],y=w*A[j+mid+k];
				A[j+k]=x+y;
				A[j+mid+k]=x-y;
			}
		}
	}
}
void chen(node *A,node B[N]){
	FFT(A,1);FFT(B,1);
	for(int i=0;i<=limit;++i)A[i]=A[i]*B[i];
	FFT(A,-1);
}
int main(){
	string a1,b1;cin>>a1>>b1;
	n=a1.size(),m=b1.size();
	for(int i=0;i<n;++i){
		a[i].x=(int)(a1[i]-'0');
	}
	for(int i=0;i<m;++i){
		b[i].x=(int)(b1[i]-'0');
	}
	while(limit<=n+m)limit<<=1,l++;
	for(int i=0;i<limit;++i){
		r[i]=(r[i>>1]>>1)|((i&1)<<(l-1));
	}
//	FFT(a,1);FFT(b,1);
//	for(int i=0;i<=limit;++i)a[i]=a[i]*b[i];
//	FFT(a,-1);
	chen(a,b);
	for(int i=0;i<=n+m;++i){
		kkk[i]+=(int)(a[i].x/limit+0.5);
		if(kkk[i]>=10){
			kkk[i+1]+=kkk[i]/10;
			kkk[i]%=10;
		}
		cout<<kkk[i];
	}
	return 0;
}