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 = 1111111;
const double EPS = 1e-8;
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) < 0 || sgn(x - a.x) == 0 && 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 = 0;
    for (int i = 0; i < n; i++) {
        while (m > 1 && sgn(cross(pt[i] - ch[m - 2], ch[m - 1] - ch[m - 2])) <= 0) m--;
        ch[m++] = pt[i];
    }
    int k = m;
    for (int i = n - 2; i >= 0; i--) {
        while (m > k && sgn(cross(pt[i] - ch[m - 2], ch[m - 1] - ch[m - 2])) <= 0) m--;
        ch[m++] = pt[i];
    }
    if (n > 1) 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 < 3) return 0;
    double ans = area(pt[0], pt[1], pt[2]);
    for (int i = 0, j = 1, k = 2; i < n; i++) {
        while (true) {
            bool df = false;
            if (i == j) j = (j + 1) % n;
            if (j == k) k = (k + 1) % n;
            while (true) {
                double a1 = area(pt[i], pt[j], pt[k]);
                double a2 = area(pt[i], pt[j], pt[(k + 1) % n]);
                ans = max(ans, a1);
                if (sgn(a1 - a2) > 0) break; 
                k = (k + 1) % n;
                df = true;
            }
            while (true) {
                double a1 = area(pt[i], pt[j], pt[k]);
                double a2 = area(pt[i], pt[(j + 1) % n], pt[k % n]);
                ans = max(ans, a1);
                if (sgn(a1 - a2) > 0) break; 
                j = (j + 1) % n;
                df = true;
            }
            if (!df) break;
        }
    }
    return ans / 2;
}

int main() {
    int n;
    while (~scanf("%d", &n)) {
        for (int i = 0; i < n; i++) scanf("%lf%lf", &pt[i].x, &pt[i].y);
        n = andrew(pt, ch, n);
        printf("%.2f
", work(ch, n));
    }
    return 0;
}

——written by Lyon