hdu 3934 Summer holiday （凸包+旋转卡壳）

Problem - 3934

　　晚上为了演示给师弟看水平序的凸包是多么的好写，于是就随便找了一题凸包，25min居然1y掉了。。

代码如下：

 #include <cmath>
 #include <cstdio>
 #include <iostream>
 #include <cstring>
 #include <algorithm>

 using namespace std;

 const int N = ;
 const double EPS = 1e-;
 inline int sgn(double x) { return (x > EPS) - (x < -EPS);}
 struct Point {
     double x, y;
     Point() {}
     Point(double x, double y) : x(x), y(y) {}
     Point operator - (Point a) { return Point(x - a.x, y - a.y);}
     bool operator < (Point a) const { return sgn(x - a.x) <  || sgn(x - a.x) ==  && y < a.y;}
 } ;

 inline double cross(Point a, Point b) { return a.x * b.y - a.y * b.x;}

 int andrew(Point *pt, Point *ch, int n) {
     sort(pt, pt + n);
     int m = ;
     for (int i = ; i < n; i++) {
         while (m >  && sgn(cross(pt[i] - ch[m - ], ch[m - ] - ch[m - ])) <= ) m--;
         ch[m++] = pt[i];
     }
     int k = m;
     for (int i = n - ; i >= ; i--) {
         while (m > k && sgn(cross(pt[i] - ch[m - ], ch[m - ] - ch[m - ])) <= ) m--;
         ch[m++] = pt[i];
     }
     if (n > ) m--;
     return m;
 }

 Point ch[N], pt[N];
 inline double area(Point a, Point b, Point c) { return fabs(cross(c - a, b - a));}

 double work(Point *pt, int n) {
     if (n < ) return ;
     double ans = area(pt[], pt[], pt[]);
     for (int i = , j = , k = ; i < n; i++) {
         while (true) {
             bool df = false;
             if (i == j) j = (j + ) % n;
             if (j == k) k = (k + ) % n;
             while (true) {
                 double a1 = area(pt[i], pt[j], pt[k]);
                 double a2 = area(pt[i], pt[j], pt[(k + ) % n]);
                 ans = max(ans, a1);
                 if (sgn(a1 - a2) > ) break;
                 k = (k + ) % n;
                 df = true;
             }
             while (true) {
                 double a1 = area(pt[i], pt[j], pt[k]);
                 double a2 = area(pt[i], pt[(j + ) % n], pt[k % n]);
                 ans = max(ans, a1);
                 if (sgn(a1 - a2) > ) break;
                 j = (j + ) % n;
                 df = true;
             }
             if (!df) break;
         }
     }
     return ans / ;
 }

 int main() {
     int n;
     while (~scanf("%d", &n)) {
         for (int i = ; i < n; i++) scanf("%lf%lf", &pt[i].x, &pt[i].y);
         n = andrew(pt, ch, n);
         printf("%.2f\n", work(ch, n));
     }
     return ;
 }

——written by Lyon