uva 12121 - You are around me ...(贪心+几何）

题目链接：uva 12121 - You are around me ...

题目大意：给出n，e和th，表示有n个男孩，每个男孩有一个椭圆形的区间，离心率为e，和x轴的坐标夹角为th，没个男孩的区间大小都相同，并且没有两个男孩的区间重叠，问说椭圆形的面积最大为多少。

解题思路：将整个坐标系旋转反的th角度，然后将每个点按照x排序，遍历一遍，根据e和两点坐标求出a，维护a的最小值。然后再按照y同样做一次即可。

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <algorithm>

using namespace std;
const int N = 15005;
const double pi = 4.0 * atan(1.0);
const double INF = 0x3f3f3f3f3f3f3f3f;

struct point {
double x, y;
}p
;
int n;
double e, th, k;

inline void init () {
double xi, yi;
th = th * pi / 180;
for (int i = 0; i < n; i++) {
scanf("%lf%lf", &xi, &yi);
p[i].x = xi * cos(th) + yi * sin(th);
p[i].y = -xi * sin(th) + yi * cos(th);
}
}

inline bool cmpX (const point& a, const point& b) {
return a.x < b.x;
}

inline bool cmpY (const point& a, const point& b) {
return a.y < b.y;
}

inline double cat (point a, point b) {
double xi = fabs(a.x - b.x) / 2;
double yi = fabs(a.y - b.y) / 2;

return (k * xi * xi + yi * yi) / k;
}

double solve () {
double ans = INF;

sort (p, p + n, cmpX);

for (int t = 0; t < 2; t++) {
for (int i = 1; i < n; i++) {
double tmp = cat(p[i-1], p[i]);
ans = min (ans, tmp);
}

sort (p, p + n, cmpY);
}
return ans;
}

int main () {
int cas = 1;
while (scanf("%d%lf%lf", &n, &e, &th) == 3 && n && e && th) {
init ();

k = 1 - e * e;
double d = solve ();

printf("Case %d:\n", cas++);
if (e == 1)
printf("%.6lf\n", 0.0);
else
printf("%.6lf\n", pi * d * sqrt(k));
}
return 0;
}