水题堆3.P - Shape of HDU

#include 
#include 
int x[10000000],y[10000000];
int main()
{
    int n,i,a;

    while(1){
        scanf("%d",&n);
        if(n==0)break;
        int flag=0;
        for(i=0;i<=n-1;i++)scanf("%d%d",&x[i],&y[i]);
        for(i=0;i<=n-3;i++){
            a=(x[i+1]-x[i])*(y[i+2]-y[i])-(x[i+2]-x[i])*(y[i+1]-y[i]);
            if(a<0){
                flag=1;
                break;
            }
        }
        a=(x[n-1]-x[n-2])*(y[0]-y[n-1])-(x[0]-x[n-2])*(y[n-1]-y[n-2]);
        if(a<0)flag=1;
        a=(x[0]-x[n-1])*(y[1]-y[0])-(x[1]-x[n-1])*(y[0]-y[n-1]);
        if(a<0)flag=1;
        if(flag==1)printf("concave\n");
        else printf("convex\n");
    }
    return 0;
}

因为每个点是逆时针输入的，按A,B,C,D...这样输入，所以如果AC向量在AB向量的顺时针方向，则肯定是凹多边形(ABXAC)<0