bzoj 2179 FFT快速傅立叶
题目:https://www.lydsy.com/JudgeOnline/problem.php?id=2179
注意进位的时候最好先把所有的都变成 int 再普通进位。
#include<iostream> #include<cstdio> #include<cstring> #include<algorithm> #include<cmath> #define db double using namespace std; const int N=6e4+5,M=N<<2; const db pi=acos(-1); int n,len,r[M]; struct cpl{db x,y;int c;}a[M],b[M],I; cpl operator+ (cpl a,cpl b){return (cpl){a.x+b.x,a.y+b.y};} cpl operator- (cpl a,cpl b){return (cpl){a.x-b.x,a.y-b.y};} cpl operator* (cpl a,cpl b){return (cpl){a.x*b.x-a.y*b.y,a.x*b.y+a.y*b.x};} void fft(cpl *a,bool fx) { for(int i=0;i<len;i++) if(i<r[i])swap(a[i],a[r[i]]); for(int R=2;R<=len;R<<=1)//<= { int m=R>>1; cpl Wn=(cpl){ cos(pi/m),fx?-sin(pi/m):sin(pi/m) }; for(int i=0;i<len;i+=R) { cpl w=I; for(int j=0;j<m;j++,w=w*Wn) { cpl tmp=w*a[i+m+j]; a[i+m+j]=a[i+j]-tmp; a[i+j]=a[i+j]+tmp; } } } } int main() { scanf("%d",&n); n--; I.x=1; for(int i=n,d;i>=0;i--) { scanf("%1d",&d); a[i].x=d; } for(int i=n,d;i>=0;i--) { scanf("%1d",&d); b[i].x=d; } len=1; for(;len<=(n<<1);len<<=1); for(int i=0;i<len;i++) r[i]=(r[i>>1]>>1)+((i&1)?len>>1:0); fft(a,0); fft(b,0); for(int i=0;i<len;i++) a[i]=a[i]*b[i]; fft(a,1); int t=n<<1; for(int i=0;i<=t;i++) a[i].c=int(a[i].x/len+0.5); for(int i=0;i<t;i++)//< { a[i+1].c+=a[i].c/10; a[i].c%=10; } while(!a[t].c&&t>=0)t--; if(t<0)putchar('0'); for(int i=t;i>=0;i--) printf("%d",a[i].c); puts(""); return 0; }