bzoj2179: FFT快速傅立叶
http://www.lydsy.com/JudgeOnline/problem.php?id=2179
FFT做高精乘
#include<cmath> #include<cstdio> #include<complex> using namespace std; #define N 60001 const int M=(1<<17)+10; const double pi=acos(-1); typedef complex<double> E; int n; char s[N]; int r[M]; E a[M],b[M]; int res[M]; void fft(E *a,int f) { for(int i=0;i<n;++i) if(i<r[i]) swap(a[i],a[r[i]]); for(int i=1;i<n;i<<=1) { E wn(cos(pi/i),f*sin(pi/i)); for(int p=i<<1,j=0;j<n;j+=p) { E w(1,0); for(int k=0;k<i;++k,w*=wn) { E x=a[j+k],y=w*a[j+k+i]; a[j+k]=x+y; a[j+k+i]=x-y; } } } } int main() { int m; scanf("%d",&n); n--; m=n; scanf("%s",s); for(int i=n;i>=0;--i) a[n-i]=s[i]-'0'; scanf("%s",s); for(int i=m;i>=0;--i) b[m-i]=s[i]-'0'; m+=n; int bit=0; for(n=1;n<=m;n<<=1) bit++; for(int i=0;i<n;++i) r[i]=(r[i>>1]>>1)|((i&1)<<bit-1); fft(a,1); fft(b,1); for(int i=0;i<n;++i) a[i]=a[i]*b[i]; fft(a,-1); for(int i=0;i<=m;++i) res[i]=(int)(a[i].real()/n+0.5); for(int i=0;i<=m;++i) res[i+1]+=res[i]/10,res[i]%=10; if(res[m+1]) m++; //while(m && !res[m]) --m; for(int i=m;i>=0;--i) printf("%d",res[i]); }