HOJ 1157 SCUD Busters

题目：

    给出n个凸包，现在要用炸弹炸毁固定的坐标，被攻击的坐标所在的凸包会被毁灭，

    现在问你被毁灭的凸包的面积



分析：

    求凸包+求多边形面积

 

#include <iostream>

#include <cstdio>

#include <cstring>

#include <algorithm>

#include <cmath>



using namespace std;



const int X = 105;

#define debug puts("here");



int top,n,a[X];

bool use[X];



struct node

{

    int x,y;

    friend bool operator < (node a,node b)

    {

        return a.y<b.y||(a.y==b.y&&a.x<b.x);

    }

}res[X],p[X],in[25][X];



int det(int x1,int y1,int x2,int y2)

{

    return x1*y2-x2*y1;

}



bool del(int top,int i)

{

    if(det(res[top].x-res[top-1].x,res[top].y-res[top-1].y,p[i].x-res[top].x,p[i].y-res[top].y)<=0)

        return true;

    return false;

}



void graham()

{

    sort(p,p+n);

    top = 1;

    res[0] = p[0];

    res[1] = p[1];

    for(int i=2;i<n;i++)

    {

        while(top&&del(top,i))

            top--;

        res[++top] = p[i];

    }

    int mint = top;

    res[++top] = p[n-2];

    for(int i=n-3;i>=0;i--)

    {

        while(top!=mint&&del(top,i))

            --top;

        res[++top] = p[i];

    }

}



int det(node a,node b,node o)

{

    return (a.x-o.x)*(b.y-o.y)-(a.y-o.y)*(b.x-o.x);

}



bool check(node cur,int id)

{

    for(int i=0;i<a[id]-1;i++)

        if(det(in[id][i],in[id][i+1],cur)*det(in[id][i+1],in[id][i+2],cur)<0)

            return false;

    return true;

}



double dis(node a,node b)

{

    return sqrt((a.x-b.x)*1.0*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));

}



double cal(int id)

{

    double ans = 0;

    for(int i=1;i<a[id];i++)

        ans += det(in[id][i],in[id][i+1],in[id][0]);

    ans /= 2;

    ans = max(ans,-ans);

    return ans;

}



double solve(int x,int y,int cnt)

{

    node cur;

    cur.x = x;

    cur.y = y;

    for(int i=1;i<=cnt;i++)

        if(!use[i]&&check(cur,i))

        {

            use[i] = true;

            return cal(i);

        }

    return 0;

}



int main()

{

    memset(use,false,sizeof(use));



    int cnt = 0;

    while(cin>>n,n!=-1)

    {

        for(int i=0;i<n;i++)

            scanf("%d%d",&p[i].x,&p[i].y);

        graham();

        cnt++;

        for(int i=0;i<=top;i++)

            in[cnt][i] = res[i];

        a[cnt] = top;

    }

    int x,y;

    double ans = 0;

    while(cin>>x>>y)

        ans += solve(x,y,cnt);

    printf("%.2lf\n",ans);

    return 0;

}