poj 1228 Grandpa's Estate

#include<stdio.h>
#include<algorithm>
using namespace std;
#define Max 1000
struct Point
{
int x,y;
}p[Max],res[Max];
int xmult(Point p1,Point p2,Point p0)
{
return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}
bool cmp(Point p1,Point p2)
{
if(p1.y<p2.y||p1.y==p2.y&&p1.x<p2.x)
return true;
return false;
}
int Graham(Point *p,int n)
{
int i,len,top=1;
sort(p,p+n,cmp);
if(n==0)
{
return 0;
}
res[0]=p[0];
if(n==1)
{
return 1;
}
res[1]=p[1];
if(n==2)
{
return 2;
}
res[2]=p[2];
for(i=2;i<n;i++)
{
while(top&&xmult(res[top],p[i],res[top-1])<0)
{
top--;
}
res[++top]=p[i];
}
len=top;//判断是否只有一半
res[++top]=p[n-2];
for(i=n-3;i>=0;i--)
{
while(len!=top&&xmult(res[top],p[i],res[top-1])<0)
{
top--;
}
res[++top]=p[i];
}
return top;
}
int main()
{
int _case,n,i;
scanf("%d",&_case);
while(_case--)
{
scanf("%d",&n);
for(i=0;i<n;i++)
scanf("%d%d",&p[i].x,&p[i].y);
n=Graham(p,n);
int j=0,bj=0,ji=1;
res[n++]=res[0];
res[n++]=res[1];
for(i=1;i<n;i++)
{
if(!xmult(res[i],res[i+1],res[j]))
{
bj++;
while(!xmult(res[i],res[i+1],res[j]))i++;
j=i;
if(j!=n-2)ji++;
//printf()
//printf("#%d %d\n",ji,bj);
}
else break;
}
//printf("#%d %d\n",ji,bj);
if(ji>=3&&bj==ji)
printf("YES\n");
else
printf("NO\n");
}
return 0;
}