POJ 1039 Pipe <计算几何>

题目

分析：初次做计算几何的题，看了一下ljr的《算法艺术与信息学竞赛》，这本书第三章就是计算几何初步，可以用来入门吧。书上p.359有这道例题，看一下前面的基础知识，应该就知道如何做了。

代码:

#include <iostream>#include <iomanip>#include <algorithm>#include <cmath>using namespace std;const double precision=1e-7;const double INF=999999.0;struct Point{    double x,y;};Point up[25],down[25];int n;int dlbcmp(double x){    return fabs(x)<precision?0:(x>0?1:-1);}double det(double x1,double y1,double x2,double y2){    return x1*y2-x2*y1;}double cross(Point& a,Point& b,Point& c){    return det(b.x-a.x,b.y-a.y,c.x-a.x,c.y-a.y);}bool check(Point& a,Point& b,Point& c,Point& d){    return dlbcmp(cross(a,b,c))*dlbcmp(cross(a,b,d))<=0;//这里允许非规范相交}double intersection(Point& a,Point& b,Point& c,Point& d){    int d1,d2;    double s1,s2;    d1=dlbcmp(s1=cross(a,b,c));    d2=dlbcmp(s2=cross(a,b,d));    if(d1^d2==2){        return (s2*c.x-s1*d.x)/(s2-s1);    }    else if(!d1) return c.x;    else if(!d2) return d.x;    return -INF;}void solve(){    double ans=-INF;    for(int i=0;i<n;++i){        for(int j=0;j<n;++j){            if(i==j) continue;            int k;            for(k=0;k<n;++k){                if(!check(up[i],down[j],up[k],down[k]))                    break;            }            if(k==n){                cout<<"Through all the pipe."<<endl;                return ;            }            if(k>max(i,j)){                ans=max(intersection(up[i],down[j],up[k],up[k-1]),ans);                ans=max(intersection(up[i],down[j],down[k],down[k-1]),ans);            }        }    }    cout<<fixed<<setprecision(2)<<ans<<endl;}int main(){    ios::sync_with_stdio(false);    while(cin>>n,n){        for(int i=0;i<n;++i){            cin>>up[i].x>>up[i].y;            down[i].x=up[i].x;            down[i].y=up[i].y-1;        }        solve();    }    return 0;}