杭电2108 Shape of HDU

Submit

#include<stdio.h>
#include<string.h>
int a[10000],b[10000];
int main()
{
	int i,n,j,k,l;
	while(scanf("%d",&n)&&n!=0)
	{
		k=1;
		for(i=0;i<n;i++)
		scanf("%d%d",&a[i],&b[i]);
		a[n]=a[0];
        b[n]=b[0];
        a[n+1]=a[1];
        b[n+1]=b[1];
		for(i=0;i<n;i++)
		{
		int s=(a[i+1]-a[i])*(b[i+2]-b[i])-(a[i+2]-a[i])*(b[i+1]-b[i]);
		if(s<0)
        k=0;
		}
		if(k==1)
		printf("convex\n");
		else
		printf("concave\n");
	}
	return 0;
}