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  • hdu----(1402)A * B Problem Plus(FFT模板)

    A * B Problem Plus

    Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
    Total Submission(s): 12665    Accepted Submission(s): 2248


    Problem Description
    Calculate A * B.
     
    Input
    Each line will contain two integers A and B. Process to end of file.

    Note: the length of each integer will not exceed 50000.
     
    Output
    For each case, output A * B in one line.
     
    Sample Input
    1 2 1000 2
     
    Sample Output
    2 2000
     
    Author
    DOOM III
    快速傅里叶转换算法.....
    代码:
     1 #include <iostream>
     2 #include <cstdio>
     3 #include <cmath>
     4 #include <cstring>
     5 using namespace std;
     6 #define N 50500*2
     7 const double PI=acos(-1.0);
     8 struct Vir
     9 {
    10     double re,im;
    11     Vir(double _re=0.,double _im=0.):re(_re),im(_im){}
    12     Vir operator*(Vir r) { return Vir(re*r.re-im*r.im,re*r.im+im*r.re);}
    13     Vir operator+(Vir r) { return Vir(re+r.re,im+r.im);}
    14     Vir operator-(Vir r) { return Vir(re-r.re,im-r.im);}
    15 };
    16 void bit_rev(Vir *a,int loglen,int len)
    17 {
    18     for(int i=0;i<len;++i)
    19     {
    20         int t=i,p=0;
    21         for(int j=0;j<loglen;++j)
    22         {
    23             p<<=1;
    24             p=p|(t&1);
    25             t>>=1;
    26         }
    27         if(p<i)
    28         {
    29             Vir temp=a[p];
    30             a[p]=a[i];
    31             a[i]=temp;
    32         }
    33     }
    34 }
    35 void FFT(Vir *a,int loglen,int len,int on)
    36 {
    37     bit_rev(a,loglen,len);
    38 
    39     for(int s=1,m=2;s<=loglen;++s,m<<=1)
    40     {
    41         Vir wn=Vir(cos(2*PI*on/m),sin(2*PI*on/m));
    42         for(int i=0;i<len;i+=m)
    43         {
    44             Vir w=Vir(1.0,0);
    45             for(int j=0;j<m/2;++j)
    46             {
    47                 Vir u=a[i+j];
    48                 Vir v=w*a[i+j+m/2];
    49                 a[i+j]=u+v;
    50                 a[i+j+m/2]=u-v;
    51                 w=w*wn;
    52             }
    53         }
    54     }
    55     if(on==-1)
    56     {
    57         for(int i=0;i<len;++i){
    58           a[i].re/=len;
    59           a[i].im/=len;
    60         }
    61     }
    62 }
    63 char a[N*2],b[N*2];
    64 Vir pa[N*2],pb[N*2];
    65 int ans[N*2];
    66 int main ()
    67 {
    68     while(scanf("%s%s",a,b)!=EOF)
    69     {
    70         int lena=strlen(a);
    71         int lenb=strlen(b);
    72         int n=1,loglen=0;
    73         while(n<lena+lenb) n<<=1,loglen++;
    74         for(int i=0,j=lena-1;i<n;++i,--j)
    75             pa[i]=Vir(j>=0?a[j]-'0':0.,0.);
    76         for(int i=0,j=lenb-1;i<n;++i,--j)
    77             pb[i]=Vir(j>=0?b[j]-'0':0.,0.);
    78          memset(ans,0,sizeof(int)*(n+1));
    79         FFT(pa,loglen,n,1);
    80         FFT(pb,loglen,n,1);
    81         for(int i=0;i<n;++i)
    82             pa[i]=pa[i]*pb[i];
    83         FFT(pa,loglen,n,-1);
    84 
    85         for(int i=0;i<n;++i)
    86             ans[i]=pa[i].re+0.5;
    87         for(int i=0;i<n;++i)
    88         {
    89              ans[i+1]+=ans[i]/10;
    90              ans[i]%=10;
    91         }
    92         int pos=lena+lenb-1;
    93         for(;pos>0&&ans[pos]<=0;--pos) ;
    94         for(;pos>=0;--pos)
    95             printf("%d",ans[pos]);
    96         puts("");
    97     }
    98     return 0;
    99 }
    View Code
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  • 原文地址:https://www.cnblogs.com/gongxijun/p/3970739.html
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