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 + ;
 using namespace std;
 const int MOD = 1e9+;
 typedef long long ll;

 const double eps = 1e-;
 #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[]={,,};
     for (i=;i<n&&s[]|s[];i++)
         s[_sign(xmult(p[(i+)%n],p[(i+)%n],p[i]))] = ;
     return s[]|s[];
 }

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