HDU 2215 Maple trees

增量法的最小包围圈算法，不会……

#include <cstdio>

#include <cstring>

#include <iostream>

#include <cmath>

#include <algorithm>

using namespace std;

const double EPS = 1e-10;

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) {}

    bool operator < (Point a) const { return sgn(x - a.x) < 0 || sgn(x - a.x) == 0 && sgn(y - a.y) < 0;}

    bool operator == (Point a) const { return sgn(x - a.x) == 0 && sgn(y - a.y) == 0;}

    Point operator + (Point a) const { return Point(x + a.x, y + a.y);}

    Point operator - (Point a) const { return Point(x - a.x, y - a.y);}

    Point operator * (double p) const { return Point(x * p, y * p);}

    Point operator / (double p) const { return Point(x / p, y / p);}

} ;

typedef Point Vec;

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

inline double crossDet(Point o, Point a, Point b) { return crossDet(a - o, b - o);}

inline double dotDet(Vec a, Vec b) { return a.x * b.x + a.y * b.y;}

inline double vecLen(Vec x) { return sqrt(dotDet(x, x));}

inline Point normal(Vec x) { return Point(-x.y, x.x) / vecLen(x);}

Point lineIntersect(Point P, Vec v, Point Q, Vec w) {

    Vec u = P - Q;

    double t = crossDet(w, u) / crossDet(v, w);

    return P + v * t;

}

inline Point getMid(Point a, Point b) { return (a + b) / 2.0;}

struct Circle {

    Point c;

    double r;

    Circle() {}

    Circle(Point c, double r) : c(c), r(r) {}

} ;



Circle getCircle(Point a, Point b, Point c) {

    Vec v1 = b - a, v2 = c - a;

    if (sgn(dotDet(b - a, c - a)) <= 0) return Circle(getMid(b, c), vecLen(b - c) / 2.0);

    if (sgn(dotDet(a - b, c - b)) <= 0) return Circle(getMid(a, c), vecLen(a - c) / 2.0);

    if (sgn(dotDet(a - c, b - c)) <= 0) return Circle(getMid(a, b), vecLen(a - b) / 2.0);

    Point ip = lineIntersect(getMid(a, b), normal(v1), getMid(a, c), normal(v2));

    return Circle(ip, vecLen(ip - a));

}

int andrew(Point *pt, int n, Point *ch) {

    sort(pt, pt + n);

    int m = 0;

    for (int i = 0; i < n; i++) {

        while (m > 1 && sgn(crossDet(ch[m - 2], ch[m - 1], pt[i])) <= 0) m--;

        ch[m++] = pt[i];

    }

    int k = m;

    for (int i = n - 2; i >= 0; i--) {

        while (m > k && sgn(crossDet(ch[m - 2], ch[m - 1], pt[i])) <= 0) m--;

        ch[m++] = pt[i];

    }

    if (n > 1) m--;

    return m;

}

const int N = 555;

Point pt[N], ch[N];

int rnd[N];

void randPoint(Point *pt, int n) {

    for (int i = 0; i < n; i++) rnd[i] = (rand() % n + n) % n;

    for (int i = 0; i < n; i++) swap(pt[i], pt[rnd[i]]);

}

inline bool inCircle(Point p, Circle C) { return sgn(vecLen(C.c - p) - C.r) <= 0;}

int main() {

    int n;

    while (cin >> n && n) {

        for (int i = 0; i < n; i++) scanf("%lf%lf", &pt[i].x, &pt[i].y);

        n = andrew(pt, n, ch);

        randPoint(ch, n);

        Circle ans = Circle(ch[0], 0.0), tmp;

        for (int i = 0; i < n; i++) {

            if (inCircle(ch[i], ans)) continue;

            ans = Circle(ch[i], 0.0);

            for (int j = 0; j < i; j++) {

                if (inCircle(ch[j], ans)) continue;

                ans = Circle(getMid(ch[i], ch[j]), vecLen(ch[i] - ch[j]) / 2.0);

                for (int k = 0; k < j; k++) {

                    if (inCircle(ch[k], ans)) continue;

                    ans = getCircle(ch[i], ch[j], ch[k]);

                }

            }

        }

        printf("%.2f\n", ans.r + 0.5);

    }

    return 0;

}