初中生数学题

题意

平面上最开始只包含3个点，然后还会依次出现N个点。每新增一个点，请你求出包含这些点的周长最小的多边形的面积。

---

思路

采用平衡树维护，每次插入寻找前驱和后继，然后添加节点即可。

注意本题有可能会出现三点共线的情况，可以通过随机指定初始点来解决。

精度略有毒瘤之处。

代码

#include <bits/stdc++.h>
#include <cmath>

using namespace std;

namespace StandardIO {

	template<typename T> inline void read (T &x) {
		x=0;T f=1;char c=getchar();
		for (; c<'0'||c>'9'; c=getchar()) if (c=='-') f=-1;
		for (; c>='0'&&c<='9'; c=getchar()) x=x*10+c-'0';
		x*=f;
	}
	template<typename T> inline void write (T x) {
		if (x<0) putchar('-'),x=-x;
		if (x>=10) write(x/10);
		putchar(x%10+'0');
	}

}

using namespace StandardIO;

namespace Solve {
	
	const int N=100100;
	
	int n;
	long long ans;
	struct node {
		long long x,y;
		long double rad;
		node () {}
		node (int _x,int _y):x(_x),y(_y){}
		node (int _x,int _y,long double _r):x(_x),y(_y),rad(_r){}
		node operator - (const node &a) {
			return node(x-a.x,y-a.y);
		}
		node operator + (const node &a) {
			return node(x+a.x,y+a.y);
		} 
		long long operator * (const node &a) {
			return (x*a.y-y*a.x);
		}
	} a[N],S;
	long double X,Y;
	set<node> sexy;
	
	inline bool operator < (const node &x,const node &b) {
		return x.rad<b.rad;
	}
	inline long double rad (node x) {
		return (long double)atan2((x.y-S.y),(x.x-S.x));
	}
	inline node pre (node x) {
		if (sexy.count(x)) return x;
		set<node>::iterator it=sexy.lower_bound(x);
		if (it==sexy.begin()) it=sexy.end();
		return *(--it);
	}
	inline node suc (node x) {
		set<node>::iterator it=sexy.upper_bound(x);
		if (it==sexy.end()) it=sexy.begin();
		return *it;
	}
	inline long long Abs (long long a) {
		return (a>0)?a:-a;
	}
	inline long long calc (node x,node b) {
		return Abs(x*b);
	}
	inline void insert (node x) {
		if (sexy.size()<=2) {
			sexy.insert(x);
			return;
		}
		node l=pre(x),r=suc(x);
		if ((x-l)*(r-l)<=0) return;
		ans+=calc(x-l,r-l);
		while (1) {
			node tmp=pre(x);
			sexy.erase(tmp),l=pre(x);
			if ((x-l)*(l-tmp)>=0) {
				sexy.insert(tmp);
				break;
			}
			ans+=calc(tmp-x,l-x);
		}
		while (1) {
			node tmp=suc(x);
			sexy.erase(tmp),r=suc(x);
			if ((x-r)*(r-tmp)<=0) {
				sexy.insert(tmp);
				break;
			}
			ans+=calc(tmp-x,r-x);
		}
		sexy.insert(x);
	}
	inline void init () {
		long double J=(long double)1.92,Z=(long double)6.08,M=(long double)1.7;
		S=node((long double)((long double)a[1].x*J+(long double)a[2].x*Z+(long double)a[3].x*M)/(long double)(J+Z+M),(long double)((long double)a[1].y*J+(long double)a[2].y*Z+(long double)a[3].y*M)/(long double)(J+Z+M));
		insert(node(a[1].x,a[1].y,rad(a[1])));
		insert(node(a[2].x,a[2].y,rad(a[2])));
		insert(node(a[3].x,a[3].y,rad(a[3])));
		ans+=calc(a[1]-a[3],a[3]-a[2]);
	}

	inline void MAIN () {
		for (register int i=1; i<=3; ++i) read(a[i].x),read(a[i].y);
		init();
		read(n);
		for (register int i=4; i<=n+3; ++i) {
			read(a[i].x),read(a[i].y);
			insert(node(a[i].x,a[i].y,rad(a[i])));
			write(ans),putchar('
');
		}
	}
	
	#undef int
}

int main () {
	Solve::MAIN();
}