题目链接：hdu 3269 Convex

题目大意：给出n个点，问任选四个点可以组成多少个凸四边形。

解题思路：和uav11529的做法是一样的，只不过求的东西不一样。

#include 
   
     #include 
    
      #include 
     
       #include 
      
        #include 
       
         using namespace std; typedef long long ll; const int N = 1205; const double pi = 4 * atan(1.0); const double eps = 1e-9; int n; ll s; double r[2*N]; struct point { double x, y; }p[N]; ll Count (int d) { int c = 0, mv = 0; for (int i = 0; i < n; i++) { if (i == d) continue; double a = atan2(p[i].y-p[d].y, p[i].x-p[d].x); r[c] = a; r[c+n-1] = a + 2*pi; c++; } c = 2 * n - 2; sort(r, r + c); ll ans = 0; for (int i = 0; i < n-1; i++) { double tmp = r[i] + pi; while (tmp >





 r[mv]) mv++; ll cnt = mv - i - 1; ans = ans + cnt * (cnt-1) / 2; } return s - ans; } ll solve () { s = (n-1) * (n-2) * (n-3) / 6; ll c = s * n / 4; ll ans = 0; for (int i = 0; i < n; i++) ans += Count(i); return c - ans; } int main () { int cas; cin >> cas; while (cas--) { cin >> n; for (int i = 0; i < n; i++) cin >> p[i].x >> p[i].y; cout << solve() << endl; } return 0; }