bzoj1857: [Scoi2010]传送带(三分套三分)

　　经典题...

　　不知道怎么证单峰函数

　　三分第一个传送带的位置里套三分另一个传送带的位置，计算答案

#include<bits/stdc++.h>
#define ll long long 
using namespace std;
const int maxn=500010,inf=1e9;
const double eps=1e-6;
int ax,ay,bx,by,cx,cy,dx,dy,p,q,r;
void read(int &k)
{
    int f=1;k=0;char c=getchar();
    while(c<'0'||c>'9')c=='-'&&(f=-1),c=getchar();
    while(c<='9'&&c>='0')k=k*10+c-'0',c=getchar();
    k*=f;
}
double dis(double x1,double y1,double x2,double y2)
{return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));}
double calc(double x,double y)
{
    double lx=cx,ly=cy,rx=dx,ry=dy;
    while(fabs(rx-lx)>eps||fabs(ry-ly)>eps)
    {
        double midx1=lx+(rx-lx)/3,midy1=ly+(ry-ly)/3;
        double midx2=lx+((rx-lx)/3*2),midy2=ly+((ry-ly)/3*2);
        double dis1=dis(ax,ay,x,y)/p+dis(x,y,midx1,midy1)/r+dis(midx1,midy1,dx,dy)/q;
        double dis2=dis(ax,ay,x,y)/p+dis(x,y,midx2,midy2)/r+dis(midx2,midy2,dx,dy)/q;
        if(dis1-dis2>eps)lx=midx1,ly=midy1;
        else rx=midx2,ry=midy2;
    }
    return dis(ax,ay,x,y)/p+dis(x,y,lx,ly)/r+dis(lx,ly,dx,dy)/q;
}
int main()
{
    read(ax);read(ay);read(bx);read(by);
    read(cx);read(cy);read(dx);read(dy);
    read(p);read(q);read(r);
    double lx=ax,ly=ay,rx=bx,ry=by;
    while(fabs(rx-lx)>eps||fabs(ry-ly)>eps)
    {
        double midx1=lx+(rx-lx)/3,midy1=ly+(ry-ly)/3;
        double midx2=lx+((rx-lx)/3*2),midy2=ly+((ry-ly)/3*2);
        if(calc(midx1,midy1)-calc(midx2,midy2)>eps)lx=midx1,ly=midy1;
        else rx=midx2,ry=midy2;
    }
    printf("%.2lf",calc(lx,ly));
    return 0;
}