poj1228--稳定凸包

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
struct point{
    double x,y;
    point(double x=0,double y=0):x(x),y(y){}
}a[1001],ch[1001];
point operator - (point a,point b){
    return point(a.x-b.x,a.y-b.y);
}
int i,j,k,n,m,t;
double cross(point a,point b){
    return a.x*b.y-a.y*b.x;
}
bool cmp(point a,point b){
    return a.x<b.x||(a.x==b.x&&a.y<b.y)?1:0;
}
bool check(point a,point b){
    return a.x!=b.x||a.y!=b.y?1:0;
}
int main()
{
    scanf("%d",&t);
    for(int p=0;p<t;p++){
        m=0;
        scanf("%d",&n);
        for(i=1;i<=n;i++)scanf("%lf%lf",&a[i].x,&a[i].y);
        if(n<6){
            printf("NO\n");
            continue;
        }
        sort(a+1,a+n+1,cmp);
        for(i=1;i<=n;i++){
            while(m>1&&cross(ch[m]-ch[m-1],a[i]-ch[m-1])<=0)m--;
            ch[++m]=a[i];
        }
        k=m;
        for(i=n-1;i>=1;i--){
            while(m>k&&cross(ch[m]-ch[m-1],a[i]-ch[m-1])<=0)m--;
            ch[++m]=a[i];
        }
        for(i=1;i<m;i++){
            for(j=1;j<=n;j++)
            if(check(ch[i],a[j])&&check(ch[i+1],a[j]))
            if(((a[j].x>=ch[i].x&&a[j].x<=ch[i+1].x)||(a[j].x<=ch[i].x&&a[j].x>=ch[i+1].x))&&!cross(ch[i+1]-ch[i],a[j]-ch[i]))break;
            if(j==n+1)break;
        }
        if(i<m)printf("NO\n");else printf("YES\n");
    }
    return 0;
}