省选模版复习——FFT

来源：互联网发布：2015年老龄化数据编辑：程序博客网时间：2024/05/18 01:30
hdu1402 大整数乘法，裸FFT
#include <cmath>#include <cstdio>#include <cstring>#include <algorithm>using namespace std;const int Maxn=200005;const double PI=acos(-1);char S[Maxn],T[Maxn];int ans[Maxn],n,m,i,N;struct CP{  double x,y;  CP operator +(const CP &a)const    { return (CP){x+a.x, y+a.y}; }  CP operator -(const CP &a)const    { return (CP){x-a.x, y-a.y}; }  CP operator *(const CP &a)const    { return (CP){x*a.x-y*a.y, x*a.y+y*a.x}; }} A[Maxn],B[Maxn];void FFT(CP A[],int N,int flag){  for (int i=1,j=0;i<N-1;i++){  for (int s=N; (~j)&s;j^=(s>>=1));  if (i<j) swap(A[i],A[j]);  }  for (int i=2;i<=N;i<<=1){  CP wn = (CP){ cos(2*PI/i), flag*sin(2*PI/i) };  for (int j=0;j<N;j+=i){    CP w = (CP){1,0};    for (int k=j;k<j+i/2;k++){    CP x = A[k], y = A[k+i/2]*w;    A[k] = x+y; A[k+i/2] = x-y;    w = w*wn;    }    }  }  if (flag==1) return;  for (int i=0;i<N;i++) A[i].x/=N;}int main(){  freopen("1402.in","r",stdin);  freopen("1402.out","w",stdout);  while (~scanf("%s",S)){    scanf("%s",T);    n=strlen(S);    m=strlen(T);    if ((n==1&&S[0]=='0') || (m==1&&T[0]=='0'))      {puts("0");continue;}    for (N=2;N<n+m;N<<=1);    for (i=0;i<n;i++) A[n-i-1]=(CP){S[i]-'0',0};    for (i=n;i<N;i++) A[i]=(CP){0,0};    for (i=0;i<m;i++) B[m-i-1]=(CP){T[i]-'0',0};    for (i=m;i<N;i++) B[i]=(CP){0,0};    FFT(A,N,1); FFT(B,N,1);    for (i=0;i<N;i++) A[i]=A[i]*B[i];    FFT(A,N,-1);    memset(ans,0,sizeof(ans));    for (i=0;i<N;i++) ans[i]=(int)(A[i].x+0.5);    for (i=0;i<N;i++){      ans[i+1] += ans[i]/10;      ans[i] %= 10;    }    for (i=N-1;i>=0;i--)      if (ans[i]>0) break;    for (;i>=0;i--)      printf("%d",ans[i]);    printf("\n");  }  return 0;}
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