fft模板 HDU 1402

 // fft模板 HDU 1402

 #include <iostream>
 #include <cstdio>
 #include <cstdlib>
 #include <algorithm>
 #include <vector>
 #include <math.h>
 #include <memory.h>
 #include <bits/stdc++.h>
 using namespace std;
 #define LL long long
 typedef pair<int,int> pii;
 const LL inf = 0x3f3f3f3f;
 const LL MOD =100000000LL;
 const int N = ;
 const double eps = 1e-;
 void fre() {freopen("in.txt","r",stdin);}
 void freout() {freopen("out.txt","w",stdout);}
 inline int read() {int x=,f=;char ch=getchar();while(ch>''||ch<'') {if(ch=='-') f=-; ch=getchar();}while(ch>=''&&ch<='') {x=x*+ch-'';ch=getchar();}return x*f;}

 const double PI = acos(-1.0);
 //复数结构体
 struct Complex{
     double r,i;
     Complex(double _r = 0.0,double _i = 0.0){
         r = _r; i = _i;
     }
     Complex operator +(const Complex &b){
         return Complex(r+b.r,i+b.i);
     }
     Complex operator -(const Complex &b){
         return Complex(r-b.r,i-b.i);
     }
     Complex operator *(const Complex &b){
         return Complex(r*b.r-i*b.i,r*b.i+i*b.r);
     }
 };
 /*
  * 进行FFT和IFFT前的反转变换。
  * 位置i和 （i二进制反转后位置）互换
  * len必须去2的幂
  */
 void change(Complex y[],int len){
     int i,j,k;
     for(i = , j = len/;i < len-; i++){
         if(i < j)swap(y[i],y[j]);
         //交换互为小标反转的元素，i<j保证交换一次
         //i做正常的+1，j左反转类型的+1,始终保持i和j是反转的
         k = len/;
         while( j >= k){
             j -= k;
             k /= ;
         }
         if(j < k) j += k;
     }
 }
 /*
  * 做FFT
  * len必须为2^k形式，
  * on==1时是DFT，on==-1时是IDFT
  */
 void fft(Complex y[],int len,int on){
     change(y,len);
     for(int h = ; h <= len; h <<= ){
         Complex wn(cos(-on**PI/h),sin(-on**PI/h));
         for(int j = ;j < len;j+=h){
             Complex w(,);
             for(int k = j;k < j+h/;k++){
                 Complex u = y[k];
                 Complex t = w*y[k+h/];
                 y[k] = u+t;
                 y[k+h/] = u-t;
                 w = w*wn;
             }
         }
     }
     if(on == -)
         for(int i = ;i < len;i++)
             y[i].r /= len;
 }

 Complex a[N],b[N];
 char s1[N/],s2[N/];
 int ans[N];
 int main(){
     while(~scanf("%s%s",s1,s2)){
          int len1=strlen(s1);
          int len2=strlen(s2);
          int l1=,l2=;
          while((<<l1)<len1) l1++;
          while((<<l2)<len2) l2++;
          int len=(<<(max(l1,l2)+));
          for(int i=;i<len;i++){
             if(i<len1) a[i]=Complex(s1[len1-i-]-'',);
             else a[i]=Complex(,);
             if(i<len2) b[i]=Complex(s2[len2-i-]-'',);
             else b[i]=Complex(,);
          }
          fft(a,len,);
          fft(b,len,);
          for(int i=;i<len;i++)
             a[i]=a[i]*b[i];
          fft(a,len,-);
          for(int i=;i<len;i++)
             ans[i]=(int)(a[i].r+0.5);
          for(int i=;i<len-;i++){
             ans[i+]+=ans[i]/;
             ans[i]%=;
          }
          bool flag=false;
          for(int i=len-;i>=;i--){
             if(ans[i]) printf("%d",ans[i]),flag=true;
             else if(flag||i==)printf("");
         }
         printf("\n");
     }
     return ;
 }