hdu 2018 简单的凸包思想应用

  本来只要知道逆时针下，每个新增点的拐向都为逆时针就很容易做的说，不过自己太粗心………………首先拐向判断写错

然后又少判断第一个顶点这个角…………

#include<iostream>using namespace std;class cor{public:int x,y;};bool drection(cor& p0,cor& p1,cor& p2){int flag = (p2.x-p0.x)*(p1.y-p0.y)-(p1.x-p0.x)*(p2.y-p0.y);if(flag > 0)return false;return true;}int main(){int n;while(cin >>n && n){cor p0,p1,p2;cor p_0,p_1;bool flag = true;cin >>p0.x >>p0.y >>p1.x >>p1.y;p_0 = p0; p_1 = p1;for(int i = 3;i <= n;i++){cin >>p2.x >>p2.y;if(!drection(p0,p1,p2) && flag){flag = false;}p0 = p1; p1 = p2;}if(drection(p1,p2,p_0) && drection(p2,p_0,p_1))cout <<"convex" <<endl;elsecout <<"concave" <<endl;}return 0;}