题意：
给n条线段和初始点，求初始点发出的射线最多能穿过多少条线段（有交点就算穿过）。
分析：
枚举线段端点，，判断线段是否规范相交，需要严密的模板。
代码：
//poj 4048//sep9#include <iostream>using namespace std;typedef long long ll;const ll maxN=2000;const ll maxL=40028;struct P{ll x,y;}a[maxN],b[maxN],p;ll tx,ty;int n;ll det(P a,P b,P c){ll x1=b.x-a.x;ll y1=b.y-a.y;ll x2=c.x-a.x;ll y2=c.y-a.y;return x1*y2-x2*y1;}int get_sign(ll x){if(x==0) return 0;return x>0?1:-1;}ll mymax(ll a,ll b){return a>b?a:b;}ll mymin(ll a,ll b){return a>b?b:a;}int between(P mid,P a,P b){return mid.x<=max(a.x,b.x)&&mid.y<=max(a.y,b.y)&&mid.x>=min(a.x,b.x)&&mid.y>=min(a.y,b.y); }bool crossed(P a,P b,P c,P d){ int d1,d2,d3,d4; d1=get_sign(det(a,b,c)); d2=get_sign(det(a,b,d)); d3=get_sign(det(c,d,a)); d4=get_sign(det(c,d,b)); if((d1^d2)==-2&&(d3^d4)==-2)return true;if((d1==0&&between(c,a,b))||(d2==0&&between(d,a,b))||(d3==0&&between(a,c,d))||(d4==0&&between(b,c,d)))return true;return false;}int process(P dir){int cnt=0;dir.x=p.x+(dir.x-p.x)*10000;dir.y=p.y+(dir.y-p.y)*10000;if(dir.x==p.x&&dir.y==p.y) return 0;for(int i=1;i<=n;++i)if(crossed(p,dir,a[i],b[i]))++cnt;return cnt;}int main(){int i,cases;scanf("%d",&cases);while(cases–){scanf("%d",&n);for(i=1;i<=n;++i)scanf("%lld%lld%lld%lld",&a[i].x,&a[i].y,&b[i].x,&b[i].y);scanf("%lld%lld",&p.x,&p.y);int ans=0;for(i=1;i<=n;++i){ans=max(ans,process(a[i]));ans=max(ans,process(b[i]));}printf("%d\n",ans);}return 0;}
可笑的小心谨慎，还有从来就不会安全的安全感。
