$P1991 无线通讯网$

(problem)

#include <bits/stdc++.h>
#define Max(a,b) a>b?a:b
using namespace std ;
typedef long long LL ;
inline LL read() { LL x(0); register char c=getchar();
	while(!isdigit(c)) c=getchar();
	while(isdigit(c)) x=(x<<1)+(x<<3)+(c&15),c=getchar(); return x;
}
using namespace std;
const int N=505;
struct P1{
    int t1,t2;
	double v;
}line[N*N];
struct po{
    double x,y;
}st[N];
int s,p,num,fa[N];double ans;
inline double dis(int x1,int y1){
    return sqrt((st[x1].x-st[y1].x)* (st[x1].x-st[y1].x)+ (st[x1].y-st[y1].y)* (st[x1].y-st[y1].y));
}
inline bool cmp(P1 x,P1 y){ return x.v<y.v; }
inline int find(int x){ return fa[x] == x ? fa[x] : fa[x] = find (fa[x]) ; }
inline void unionn(int x,int y) { fa[find(x)] = find(y); }
int main(){
    s = read() ; p = read() ;
    for(register int i=1;i<=p;i++) fa[i] = i ;
    for(register int i=1;i<=p;i++) st[i].x = read() , st[i].y = read();
    for(register int i=1;i<=p;i++)
	    for(register int j=i+1;j<=p;j++){
	        num++;
	        line[num].t1=i; line[num].t2=j;
	        line[num].v=dis(i,j);
	    }
    sort(line+1,line+num+1,cmp);
    int a,b; s=p-s;
    for(register int i=1;i<=num,s>0;i++){
        a=line[i].t1; b=line[i].t2;
        int fx = find(a) , fy = find(b) ;
        if(fx != fy){
            unionn(a,b);
			s--;
            ans=Max(ans,line[i].v);    
        }
    }
    cout << fixed << setprecision(2) << ans << endl ;//printf("%.2lf",ans);
    return 0;
}