2017 Wuhan University Programming Contest 现场赛 K.Wifi Relay（最短路变形）

#include<bits/stdc++.h>
using namespace std;

#define maxn 10010
#define min(a,b) (a<b?a:b)
#define max(a,b) (a>b?a:b)
typedef long long LL;
const LL inf=1ll<<62;

struct point
{
LL x,y;
} pos[maxn];
LL dis[maxn],vis[maxn];
LL n;

LL sqr(LL x)
{
return x*x;
}

LL get_dis(point a,point b)
{
return sqr(a.x-b.x)+sqr(a.y-b.y);
}

LL update(LL id)//寻找最近的点
{
LL current=inf,res=-1;
for(LL i=0; i<n; ++i)
{
if(vis[i]) continue;
dis[i]=min(dis[i],get_dis(pos[id],pos[i]));
if(dis[i]<current)
{
current=dis[
4000
i];
res=i;
}
}
return res;
}

int main()
{
scanf("%lld",&n);
for(LL i=0; i<n; ++i)
scanf("%lld%lld",&pos[i].x,&pos[i].y);
memset(dis,0x3f,sizeof(dis));
LL ans=0,last=0;
vis[0]=1,dis[0]=0;
for(LL i=1; i<n; ++i)
{
last=update(last);
ans=max(ans,dis[last]);
vis[last]=1;
}
printf("%.12lf\n",sqrt(ans)/2);
return 0;
}