2012 Multi-University Training Contest 2

与六题擦边，最终五题，还有待提高。。。。

就讲讲我最后悔的那题吧（其他还不会- -！）

hdu 4316，方法的话题解已经说了

然后我犯了一个无比愚蠢的错误，把天花板的高度写成了200！！坑爹有木有啊

就这样与六题擦肩而过

#include <cmath>
#include <cstring>
#include <cstdio>
#include <algorithm>
using namespace std;
const  double INF = 1000000000.0;
const  int maxn = 1010;
const double eps = 1e-8;
inline int sgn(double x) {return fabs(x)<eps?0:(x>0?1:-1);}
struct Point{
    double x,y;
    Point(double tx=0,double ty=0){x=tx;y=ty;}
    bool operator == (const Point& t) const {
        return sgn(x-t.x)==0 && sgn(y-t.y)==0;
    }
}p[maxn],st[maxn],pp[maxn],points[maxn];
struct Seg{Point s,e;};
double Cross(Point a,Point b,Point c){
    return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);
}
/***** 凸包 ****************************************************************************/
bool cmpyx(Point a, Point b) {                                        
    if ( fabs(a.y - b.y)>eps )
        return a.y +eps< b.y;
    return a.x +eps < b.x;
}
void Grahamxy(Point *p, int &n) {                                    // 水平序(住:两倍空间)
    if ( n < 3 ) 
        return;
    int i, m=0, top=1;
    sort(p, p+n, cmpyx);    
    for (i=n; i < 2*n-1; i++)
        p[i] = p[2*n-2-i];
    for (i=2; i < 2*n-1; i++) {
        while ( top > m && Cross(p[top], p[i], p[top-1]) < eps ) 
            top--;
        p[++top] = p[i];
        if ( i == n-1 )    m = top;
    }
    n = top;
}
//半平面交
bool outside(Seg seg,Point p){return Cross(seg.s,seg.e,p)>eps;}//strictly outside the ploygon
bool inside(Seg seg,Point p){return Cross(seg.s,seg.e,p)<-eps;}//strictly inside the ploygon
Point Intersect(Point p1, Point p2, Point p3, Point p4) {
    double a1, b1, c1, a2, b2, c2, d;Point p;
    a1 = p1.y - p2.y; b1 = p2.x - p1.x; c1 = p1.x*p2.y - p2.x*p1.y;
    a2 = p3.y - p4.y; b2 = p4.x - p3.x; c2 = p3.x*p4.y - p4.x*p3.y;
    d = a1*b2 - a2*b1;
    if ( fabs(d) < eps )    return false;
    p.x = (-c1*b2 + c2*b1) / d;
    p.y = (-a1*c2 + a2*c1) / d;
    return p;
}
int n,ban_tot;
void CUT(Seg seg){//points must be clockwise        
    int i,tot=0;
    for(i=1;i<=ban_tot;i++){
        if(!outside(seg,p[i]))  pp[++tot]=p[i];//p[i] :inside or on the boundary
        else {//p[i]: must be outside 
            if(inside(seg,p[i-1]))     pp[++tot]=Intersect(seg.s,seg.e,p[i],p[i-1]);//p[i-1] must be inside
            if(inside(seg,p[i+1]))     pp[++tot]=Intersect(seg.s,seg.e,p[i],p[i+1]);//p[i+1] must be inside
        }
    }ban_tot=tot;
    pp[tot+1]=pp[1];pp[0]=pp[tot];
    memcpy(p,pp,sizeof(pp));
}
double getarea(Point *p,int n){
    int i;
    double area=0;
    for(i=0;i<n;i++) area+=p[(i+1)%n].y*(p[i].x-p[(i+2)%n].x);
    return fabs(area/2);
}
struct MACHINE{
    double x,y,z;
}mac[maxn],cam[5];
Seg ts[20000];
Point tp[maxn];
int main(){
    int n;
    while(scanf("%d",&n)!=EOF){
        for(int i=1;i<=n;i++)  scanf("%lf%lf%lf",&mac[i].x,&mac[i].y,&mac[i].z);
        for(int i=1;i<=3;i++){
            scanf("%lf%lf",&cam[i].x,&cam[i].y);
            cam[i].z=100;
        }
        int cnt=0;
        int num=0;
        p[1]=Point(-1e10,1e10);
        p[2]=Point(1e10,1e10);
        p[3]=Point(1e10,-1e10);
        p[4]=Point(-1e10,-1e10);
        p[5]=p[1];
        ban_tot=4;
        for(int i=1;i<=3;i++){
            cnt=-1;
            for(int j=1;j<=n;j++){
                double x1=cam[i].x,y1=cam[i].y,z1=cam[i].z;
                double x2=mac[j].x,y2=mac[j].y,z2=mac[j].z;
                st[++cnt].x=x1-((x2-x1)/(z2-z1)*z1);
                st[cnt].y  =y1-((y1-y2)/(z1-z2)*z1);
            }
            cnt++;
            Grahamxy(st,cnt);
            int j=0;
            for(int k=cnt-1;k>=0;k--){
                tp[++j]=st[k];
            }
            tp[cnt+1]=tp[1];
            for(int k=1;k<=cnt;k++) ts[k].s=tp[k],ts[k].e=tp[k+1];
            for(int k=1;k<=cnt;k++) CUT(ts[k]);
        }
        if(ban_tot>=3) {
            printf("%.2lf\n",getarea(p,ban_tot));
        }
        else printf("0.00");
    }
    return 0;
}