【分治】【bzoj 2458】: [BeiJing2011]最小三角形

http://www.lydsy.com/JudgeOnline/problem.php?id=2458

和最近点对一样

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;
#define rep(i,l,r) for(int i=(l),_=(r);i<=_;i++)
#define per(i,r,l) for(int i=(r),_=(l);i>=_;i--)
#define MS(arr,x) memset(arr,x,sizeof(arr))
#define INE(i,u) for(int i=head[u];~i;i=e[i].next)
#define LL long long
inline const int read()
{int r=0,k=1;char c=getchar();for(;c<'0'||c>'9';c=getchar())if(c=='-')k=-1;
for(;c>='0'&&c<='9';c=getchar())r=r*10+c-'0';return k*r;}
////////////////////////////////////////////////
const int N=200010;
const double inf=1e20;
int n;
struct Point{
int x,y;
}a
,b
;
////////////////////////////////////////////////
bool cmpx(const Point &A,const Point &B){return A.x<B.x;}
bool cmpy(const Point &A,const Point &B){return A.y<B.y;}
double sqr(double x){return x*x;}
double dis(const Point &A,const Point &B){return sqrt(sqr(A.x-B.x)+sqr(A.y-B.y));}
void MIN(double &a,double b){if(a>b)a=b;}
double cal(int l,int r)
{
if(l+1>=r) return inf;
int mid=l+r>>1;
double r1=cal(l,mid),r2=cal(mid+1,r);
double res=min(r1,r2);
double d=res/2;
int cnt=0;
rep(i,l,r) if(fabs(a[mid].x-a[i].x)<=d) b[++cnt]=a[i];
sort(&b[1],&b[cnt+1],cmpy);
rep(i,1,cnt) rep(j,i+1,cnt)
{
if(b[j].y-b[i].y>=d) break;
rep(k,j+1,cnt)
{
if(b[k].y-b[j].y>=d) break;
MIN(res,dis(b[i],b[j])+dis(b[j],b[k])+dis(b[k],b[i]));
}
}
return res;
}
////////////////////////////////////////////////
void input()
{
rep(i,1,n=read()) a[i].x=read(), a[i].y=read();
}
void solve()
{
sort(&a[1],&a[n+1],cmpx);
printf("%.6f\n",cal(1,n));
}
////////////////////////////////////////////////
int main()
{
//freopen("_.in","r",stdin); freopen("_.out","w",stdout);
input(),solve();
return 0;
}