spoj VFMUL FFT快速傅立叶变换模板题

　　题意；求两个数相乘。

　　第一次写非递归的fft，因为一个数组开小了调了两天TAT。

#include<iostream>
#include<cstring>
#include<algorithm>
#include<cstdio>
#include<cmath>
using namespace std;
#define PI 3.1415926535897932384
#define MAXN 1200000
#pragma optimize("O2")
struct Complex
{
        double x,y;
        Complex(){};
        Complex(double x,double y):x(x),y(y){};
        void init(double x,double y)
        {
                this->x=x;this->y=y;
        }
        Complex operator +(Complex a)
        {
                Complex ret(x + a.x, y + a.y);
                return ret;
        }
        Complex operator -(Complex a)
        {
                Complex ret(x - a.x, y - a.y);
                return ret;
        }
        Complex operator *(Complex a)
        {
                Complex ret(x*a.x - y*a.y, x*a.y + y*a.x);
                return ret;
        }
        Complex operator /(double v)
        {
                Complex ret(x/v,y/v);
                return ret;
        }
        Complex operator =(double x)
        {
                this->x=x;
                return *this;
        }
        void operator -=(Complex a)
        {
                this->x-=a.x;
                this->y-=a.y;
        }
        void operator +=(Complex a)
        {
                this->x+=a.x;
                this->y+=a.y;
        }
};
char ss[MAXN];
int t1[MAXN],t2[MAXN];
int pp[MAXN];
Complex ww[MAXN][];
Complex g1[MAXN],g2[MAXN];
long long res[MAXN];
int n;
void dft(Complex g[],int len,int sign)
{
        Complex t;
        for (int i=;i<len;i<<=)
        {
                for (int j=;j<len;j+=(i<<))
                {
                        for (int k=;k<i;k++)
                        {
                                t=g[j+k];
                                g[j+k]=g[j+k]+g[j+k+i]*ww[k*((n>>)/i)][sign];
                                g[j+k+i]=t-g[j+k+i]*ww[k*((n>>)/i)][sign];
                        }
                }
        }
}

int main()
{
        freopen("input.txt","r",stdin);
        //    freopen("output.txt","w",stdout);
        int i,j,k,x,y,z,m;
        int n1,n2;
        int nn;
        scanf("%d",&nn);
        while (nn--)
        {

                scanf("%s",ss);
                m=strlen(ss);
                for (i=;i<m;i++)
                        t1[(m-i-)/]=t1[(m-i-)/]*+ss[i]-'';
                n1=(m-)/+;
                scanf("%s",ss);
                m=strlen(ss);
                for (i=;i<m;i++)
                        t2[(m-i-)/]=t2[(m-i-)/]*+ss[i]-'';
                n2=(m-)/+;
                n=max(n1,n2);
                while (n!=(n&(-n)))n-=n&(-n);
                n<<=;
                for (i=;i<n;i++)
                        for (j=n>>,x=;j;j>>=,x<<=)
                                if (i&j)pp[i]+=x;
                for (i=;i<n1;i++)g1[pp[i]].x=t1[i];
                for (i=;i<n2;i++)g2[pp[i]].x=t2[i];
                for (i=;i<=n;i++)
                {
                        ww[i][].init(cos(*PI*i/n), -sin(*PI*i/n));
                        ww[i][].x=ww[i][].x;
                        ww[i][].y=-ww[i][].y;
                }
                dft(g1,n,);
                dft(g2,n,);
                for (i=;i<n;i++)g2[i]=g1[i]*g2[i];
                for (i=;i<n;i++)g1[pp[i]]=g2[i];
                dft(g1,n,);
                for (i=;i<n;i++)res[i]=(long long)(g1[i].x/n+0.5);
                for (i=;i<n;i++)res[i+]+=res[i]/,res[i]%=;
                m=n;
                while (m && !res[m-])m--;
                printf("%d",(int)res[m-]);
                for (i=m-;i>=;i--)
                        printf("%05d",(int)res[i]);
                printf("\n");
                memset(t1,,sizeof(t1[])*n);
                memset(t2,,sizeof(t2[])*n);
                memset(pp,,sizeof(pp[])*n);
                memset(res,,sizeof(res[])*n);
                memset(g1,,sizeof(g1[])*n);
                memset(g2,,sizeof(g2[])*n);
        }
}