我使用了 FFT 算法，但是无法通过本题（已通过 P1919）

#include<bits/stdc++.h>
#define int long long
using namespace std;
const double PI=acos(-1);
inline int read(){
	int x=0,w=1;
	char ch=0;
	while(ch<'0'||ch>'9'){
		if(ch=='-')w=-1;
		ch=getchar();
	}
	while(ch>='0'&&ch<='9'){
		x=x*10+(ch-'0');
		ch=getchar();
	}
	return x*w;
}
void write(int x){
	if(x<0){
		putchar('-');
		x=-x;
	}
	static int sta[35];
	int top=0;
	do{
		sta[top++]=x%10,x/=10;
	}while(x);
	while(top)putchar(sta[--top]+'0');
}
struct comp{
	double r,i;
	comp(){
		r=0,i=0;
	}
	comp(double real,double imag):r(real),i(imag){}
};
comp operator +(comp a,comp b){
	return comp(a.r+b.r,a.i+b.i);
}
comp operator -(comp a,comp b){
	return comp(a.r-b.r,a.i-b.i);
}
comp operator *(comp a,comp b){
	return comp(a.r*b.r-a.i*b.i,a.r*b.i+a.i*b.r);
}
int rev[3000005],len,lim=1;
comp a[3000005],b[3000005];
string s1,s2;
void FFT(comp *a,int op){
    for(int i=0;i<lim;i++){
        if(i<rev[i])swap(a[i],a[rev[i]]);
    }
    for(int mid=1; mid<lim; mid<<=1){
        comp wn(cos(PI/mid),op*sin(PI/mid));
        for(int r=mid<<1,j=0; j<lim; j+=r){
            comp w(1,0);
            for(int l=0; l<mid; l++,w=w*wn){
                comp x=a[j+l],y=w*a[j+mid+l];
                a[j+l]=x+y; a[j+mid+l]=x-y;
            }
        }
    }
}
int n1,n2;
signed main(){
	cin>>s1;
	n1=s1.size()-1;
	for(int i=0;i<=n1;i++)a[n1-i].r=s1[i]-'0';
	cin>>s2;
	n2=s2.size()-1;
	for(int i=0;i<=n2;i++)b[n2-i].r=s2[i]-'0';
	while(lim<=n1+n2){
		lim<<=1;len++;
	}
	for(int i=0;i<lim;i++){
		rev[i]=(rev[i>>1]>>1)|((i&1)<<(len-1));
	}
	FFT(a,1);FFT(b,1);
	for(int i=0;i<=lim;i++)a[i]=a[i]*b[i];
	FFT(a,-1);
	memset(rev,0,sizeof(rev));
	for(int i=0;i<=n1+n2+1;i++){
		rev[i]=a[i].r/lim+0.5;
	}
	for(int i=0;i<=n1+n2;i++){
		rev[i+1]+=rev[i]/10;
		rev[i]%=10;
	}
	lim=n1+n2+1;
	while(rev[lim]){
		rev[lim+1]+=rev[lim]/10;
		rev[lim]%=10;
		lim++;
	}
	for(int i=lim-1;i>=0;i--)cout<<rev[i];
	return 0;
}