hdu 2108 Shape of HDU（判定是不是凸多边形）

#include <iostream>
#include <algorithm>
#include <cstring>
#include <cstdio>
#include <string>
#include <map>
#include <cmath>
#include <vector>

#define Faster ios::sync_with_stdio(false),cin.tie(0)
#define Read freopen("in.txt","r",stdin),freopen("out.txt","w",stdout)
#define Close fclose(stdin),fclose(stdout)
const int maxn = 1e5 + 5;
using namespace std;
const int MOD = 1e9+7;
typedef long long ll;

const double eps = 1e-6;
#define _sign(x)((x)>eps?1:((x)<-eps?2:0))


struct point
{
    double x,y;
}p[maxn];

double xmult(point p1,point p2,point p0) { 
    return(p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y); 
}

int is_convex(int n) { 
    int i,s[3]={1,1,1}; 
    for (i=0;i<n&&s[1]|s[2];i++) 
        s[_sign(xmult(p[(i+1)%n],p[(i+2)%n],p[i]))] = 0; 
    return s[1]|s[2]; 
}

int main(){
    Faster;
    int n;
    while(cin >> n){
        if(n == 0)
            break;
        for(int i = 0;i < n;i++){
            cin >> p[i].x >> p[i].y;
        }
        if(is_convex(n)){
            cout << "convex" << endl;
        }
        else
            cout << "concave" << endl;
    }
    return 0;
}