POJ 1696 Space Ant

Posted 2020-06-08

极角排序

每次选择一个最外围的没选过的点，选择的时候需要利用极角排序进行选择

#include<cstdio>
#include<cstring>
#include<vector>
#include<cmath>
#include<queue>
#include<list>
#include<algorithm>
using namespace std;

const double eps=1e-8;
struct point
{
    int x,y;
    double alpha;
    int len2;
    int id;
} p[100];
int T,n;
vector<point>v;
vector<int>ans;
bool flag[100];

bool cmp(const point &a, const point &b)
{
    if(fabs(a.alpha-b.alpha)<eps) return a.len2<b.len2;
    return a.alpha<b.alpha;
}

int len2(point a,point b)
{
    return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);
}

double f(point a,point b,point c) //返回点a所对应的角的弧度
{
    double B2=(double)len2(a,c);
    double C2=(double)len2(a,b);
    double A2=(double)len2(b,c);
    double COSA=(B2+C2-A2)/(2*sqrt(B2)*sqrt(C2));
    return 3.1415926-acos(COSA);
}

int main()
{
    scanf("%d",&T);
    int Miny=999999;
    while(T--)
    {
        scanf("%d",&n);
        for(int i=1; i<=n; i++)
        {
            scanf("%d%d%d",&p[i].id,&p[i].x,&p[i].y);
            Miny=min(Miny,p[i].y);
        }

        memset(flag,0,sizeof flag);
        ans.clear();

        point pre;pre.x=-100;pre.y=Miny;
        point now;now.x=0;now.y=Miny;

        for(int i=1; i<=n; i++)
        {
            v.clear();
            for(int k=1; k<=n; k++)
            {
                if(!flag[p[k].id])
                {
                    p[k].alpha=f(now,p[k],pre);
                    p[k].len2=len2(p[k],now);
                    v.push_back(p[k]);
                }
            }
            sort(v.begin(),v.end(),cmp);
            flag[v[0].id]=1;
            ans.push_back(v[0].id);
            pre.x=now.x;
            pre.y=now.y;
            now.x=v[0].x;
            now.y=v[0].y;
        }
        printf("%d ",ans.size());
        for(int i=0; i<ans.size(); i++)
        {
            printf("%d",ans[i]);
            if(i<ans.size()-1) printf(" ");
            else printf("\n");
        }
    }
    return 0;
}