HDU1402 A * B Problem Plus FFT

#include <stdio.h>
#include <iostream>
#include <algorithm>
#include <string.h>
#include <vector>
#include <math.h>
#include <map>
using namespace std;
typedef long long LL;
const int INF=0x3f3f3f3f;
const int N=2e5+5;
const double pi = acos(-1.0);
struct complex{
  double r,i;
  complex(double R=0,double I=0){
    r=R;i=I;
  }
  complex operator+(const complex &a)const{
    return complex(r+a.r,i+a.i);
  }
  complex operator-(const complex &a)const{
    return complex(r-a.r,i-a.i);
  }
  complex operator*(const complex &a)const{
    return complex(r*a.r-i*a.i,r*a.i+i*a.r);
  }
};
void change(complex x[],int len){
   int i,j,k;
   for(i=1,j=len/2;i<len-1;++i){
     if(i<j)swap(x[i],x[j]);
     k=len/2;
     while(j>=k){j-=k;k>>=1;}
     if(j<k)j+=k;
   }
}
void fft(complex x[],int len,int on){
   change(x,len);
   for(int i=2;i<=len;i<<=1){
    complex wn(cos(-on*2*pi/i),sin(-on*2*pi/i));
    for(int j=0;j<len;j+=i){
      complex w(1,0);
      for(int k=j;k<j+i/2;++k){
          complex u = x[k];
          complex t = w*x[k+i/2];
          x[k]=u+t;
          x[k+i/2]=u-t;
          w=w*wn;
      }
    }
   }
   if(on==-1)for(int i=0;i<len;++i)x[i].r/=len;
}
complex x1[N],x2[N];
char str1[N/2],str2[N/2];
int sum[N];
int main(){  
  while(~scanf("%s%s",str1,str2)){
      int len1 = strlen(str1);
      int len2 = strlen(str2),len=1;
      while(len<len1*2||len<len2*2)len<<=1;
      for(int i=0;i<len1;++i)
        x1[i]=complex(str1[len1-1-i]-‘0‘,0);
      for(int i=len1;i<len;++i)
        x1[i]=complex(0,0);
      for(int i=0;i<len2;++i)
        x2[i]=complex(str2[len2-1-i]-‘0‘,0);
      for(int i=len2;i<len;++i)
        x2[i]=complex(0,0);
      fft(x1,len,1);
      fft(x2,len,1);
      for(int i=0;i<len;++i)
        x1[i]=x1[i]*x2[i];
      fft(x1,len,-1);
      for(int i=0;i<len;++i)
        sum[i]=(int)(x1[i].r+0.5);
      for(int i=0;i<len;++i){
        sum[i+1]+=sum[i]/10;
        sum[i]%=10;
      }
      len = len1+len2-1;
      while(sum[len]<=0&&len>0)--len;
      for(int i=len;i>=0;--i)printf("%d",sum[i]);
      printf("\n");
  }
  return 0;
}