http://acm.hdu.edu.cn/showproblem.php?pid=2108
#include<cstdio> #include<cstring> #include<iostream> #include<cmath> #include<algorithm> using namespace std; struct point { double x,y; }; bool mult(point sp,point ep,point op){ return (sp.x-op.x)*(ep.y-op.y)>=(ep.x-op.x)*(sp.y-op.y); } bool operator < (const point &l,const point &r){ return l.y<r.y || (l.y==r.y && l.x < r.x); } int graham(point pnt[],int n,point res[]){ //pnt是图中的所有的点,res是通过判断后在凸边行边上的点,而且这些点都是按逆时针存储的,n是所有点的个数 int i,len,k = 0,top = 1; sort(pnt,pnt+n); if(n == 0) return 0; res[0]=pnt[0]; if(n == 1) return 1; res[1]=pnt[1]; if(n == 2) return 2; res[2]=pnt[2]; for(i=2;i<n;i++) { while(top && mult(pnt[i],res[top],res[top-1])) top--; res[++top] = pnt[i]; } len = top; res[++top] = pnt[n-2]; for(i=n-3;i>=0;i--){ while(top!=len && mult(pnt[i],res[top],res[top-1])) top--; res[++top]=pnt[i]; } return top; // 返回凸包中点的个数 } point res[50001],pnt[50001]; int main() { int n; while(~scanf("%d",&n) && n) { for(int i=0;i<n;i++) scanf("%lf%lf",&pnt[i].x,&pnt[i].y); if(graham(pnt,n,res) == n) puts("convex"); else puts("concave"); } return 0; }