Hust oj 1642 多边形-1（叉乘）

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
using namespace std;

const int Maxn = 1005;
int x[Maxn];
int y[Maxn];

int cross(int x0,int y0,int x1,int y1,int x2,int y2)
{
int re = (x2 - x0) * (y1 - y0) - (x1 - x0) * (y2 - y0);
return re;
}

int main()
{
int n;
while(~scanf("%d",&n) && n)
{
for(int i=0;i<n;i++)
{
scanf("%d%d",&x[i],&y[i]);
}
x
= x[0];
y
= y[0];
x[n+1] = x[1];
y[n+1] = y[1];
int flag = 0;
for(int i=2;i<=n+1;i++)
{
int ans = cross(x[i-2],y[i-2],x[i-1],y[i-1],x[i],y[i]);
if(ans > 0)
{
flag = 1;
break;
}
}
if(flag)
printf("Concave\n");
else
printf("Convex\n");
}
}