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.
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 }