hdu 1007 Quoit Design(分治法求最近点对)

// Time 2093 ms; Memory 1812 K

#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
#define eps 1e-8
#define maxn 100010
#define sqr(a) ((a)*(a))

using namespace std;

int sig(double x)
{
return (x>eps)-(x<-eps);
}

struct point
{
double x,y;
point(double xx=0,double yy=0):x(xx),y(yy){}
}p[maxn];

bool operator < (point a,point b)
{
return a.x<b.x || (a.x==b.x && a.y<b.y);
}
bool cmp(point a,point b)
{
return a.y<b.y || (a.y==b.y && a.x<b.x);
}
double len(point a,point b)
{
return sqrt(sqr(a.x-b.x)+sqr(a.y-b.y));
}
double min_dis(int l,int r)
{
int i,j,k,s;
double d,mi=-1;
if(r-l<4)
{
for(i=l;i<r;i++) for(j=i+1;j<r;j++)
{
d=len(p[i],p[j]);
if(mi<0 || sig(d-mi)<0) mi=d;
}
return mi;
}
int mid=(l+r)/2;
double minl=min_dis(l,mid);
double minr=min_dis(mid,r);
mi=minl<minr?minl:minr;
for(i=1;i<mid-l && sig(p[mid].x-p[mid-i].x-mi)<0;i++);i--;
for(j=1;j<r-mid && sig(p[mid+j].x-p[mid].x-mi)<0;j++);j--;
sort(p+mid-i,p+mid,cmp);
sort(p+mid,p+mid+j+1,cmp);
int t=mid,flag;
for(k=mid-i;k<mid;k++)
{
flag=1;
for(s=t;s<=mid+j;s++)
{
if(sig(p[k].y-p[s].y)>0)
{
if(sig(p[k].y-p[s].y-mi)>=0) continue;
else
{
if(flag)
{
flag=0;t=s;
}
d=len(p[s],p[k]);
if(sig(d-mi)<0) mi=d;
}
}
else
{
if(sig(p[s].y-p[k].y-mi)>=0) break;
else
{
d=len(p[s],p[k]);
if(sig(d-mi)<0) mi=d;
}
}
}
}
sort(p+mid-i,p+mid+j+1);
return mi;
}

int main()
{
int i,n;
while(scanf("%d",&n)!=EOF && n)
{
for(i=0;i<n;i++) scanf("%lf%lf",&p[i].x,&p[i].y);
sort(p,p+n);
printf("%.2lf\n",min_dis(0,n)/2);
}
return 0;
}