luogu P1429 平面最近点对（加强版）

代码：

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

using namespace std;

const int N=200009;
const double INF=1<<30;
int n,tmp[N],cnt;
struct Point
{
	double x,y;
}a[N];

void init()
{
	scanf("%d",&n);
	for (int i=1;i<=n;i++)
		scanf("%lf %lf",&a[i].x,&a[i].y);
}

double dis(double x,double y,double X,double Y)
{
	return sqrt((x-X)*(x-X)+(y-Y)*(y-Y));
}

bool cmp(Point x,Point y)
{
	if(x.x!=y.x)
		return x.x<y.x;
	return x.y<y.y;
}

bool cmp2(int x,int y)
{
	return a[x].y<a[y].y;
}

double CDQ(int l,int r)
{
	if(l==r)
		return INF;
	if(l+1==r)
		return dis(a[l].x,a[l].y,a[r].x,a[r].y);
	int mid=l+r>>1;
	double d=min(CDQ(l,mid),CDQ(mid+1,r));
	cnt=0;
	for (int i=l;i<=r;i++)
		if(fabs(a[i].x-a[mid].x)<=d)
			tmp[++cnt]=i;
	sort(tmp+1,tmp+1+cnt,cmp2);
	for (int i=1;i<=cnt;i++)
		for (int j=i+1;j<=cnt&&a[tmp[j]].y-a[tmp[i]].y<=d;j++)
			d=min(d,dis(a[tmp[i]].x,a[tmp[i]].y,a[tmp[j]].x,a[tmp[j]].y));
	return d;
}

void work()
{
	sort(a+1,a+1+n,cmp);
	printf("%.4lf\n",CDQ(1,n));
}

int main()
{
	init();
	work();
	return 0;
}