判断多边形凹凸

任意给定一个多边形，判断它是凸还是凹。多边形的顶点以逆时针方向的序列来表示。
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#include<stdio.h>
#include<math.h>
struct point
{
	double x,y;
}g[15000];
int ploy(point a,point b,point c)
{
	int d=(c.x-a.x)*(b.y-a.y)-(b.x-a.x)*(c.y-a.y);
	if(d<=0)
	return 1;
	return 0;
}
int main()
{
	int i,j,k,m,n,ncase;
	while(scanf("%d",&ncase),ncase)
	{
		for(i=0;i<ncase;i++)
		scanf("%lf%lf",&g[i].x,&g[i].y);
		int flag=1;
		for(i=0;i<ncase;i++)
		{
			if(!ploy(g[i],g[(i+1)%ncase],g[(i+2)%ncase]))
			flag=0;
		}
		if(flag)
		printf("convex\n");
		else
		printf("concave\n");
	}
	return 0;
}