bzoj3007: 拯救小云公主

#include<cstdio>
#include<cmath>
#include<algorithm>
int n;
double xm,ym;
struct pos{double x,y;}ps[3010];
int q[3010],ed[3010];
bool discmp(double x,double y,double z){
    return x*x+y*y<z*z;
}
bool chk(double R){
    int ql=0,qr=0;
    for(int i=0;i<n;i++){
        if(ps[i].x-1<R||ps[i].y+R>ym){
            q[qr++]=i;
            ed[i]=1;
            if(ps[i].x+R>xm||ps[i].y-R<1)return 1;
        }else ed[i]=0;
    }
    while(ql!=qr){
        int w=q[ql++];
        for(int i=0;i<n;i++)if(!ed[i]&&discmp(ps[i].x-ps[w].x,ps[i].y-ps[w].y,R*2)){
            q[qr++]=i;
            ed[i]=1;
            if(ps[i].x+R>xm||ps[i].y-R<1)return 1;
        }
    }
    return 0;
}
int main(){
    scanf("%d%lf%lf",&n,&xm,&ym);
    for(int i=0;i<n;i++){
        scanf("%lf%lf",&ps[i].x,&ps[i].y);
    }
    double L=0,R=xm+ym+1;
    while(R-L>1e-5){
        double M=(L+R)/2.;
        if(chk(M))R=M;
        else L=M;
    }
    printf("%.2lf\n",(L+R)/2.);
    return 0;
}

#include<cstdio>
#include<cmath>
#include<algorithm>
int n,xm,ym,ep=0;
int f[3010];
struct pos{int x,y;}ps[3010];
struct edge{int a,b;long long c;}e[10000010];
bool operator<(edge a,edge b){
    return a.c<b.c;
}
inline long long min2(long long a,long long b){
    a*=a;b*=b;
    return a<b?a:b;
}
int get(int x){
    int a=x,c;
    while(x!=f[x])x=f[x];
    while(x!=(c=f[a]))f[a]=x,a=c;
    return x;
}
int main(){
    scanf("%d%d%d",&n,&xm,&ym);
    for(int i=0;i<n;i++){
        f[i]=i;
        scanf("%d%d",&ps[i].x,&ps[i].y);
        e[ep++]=(edge){i,n,4*min2(xm-ps[i].x,ps[i].y-1)};
        e[ep++]=(edge){i,n+1,4*min2(ps[i].x-1,ym-ps[i].y)};
        for(int j=0;j<i;j++){
            long long x=ps[i].x-ps[j].x,y=ps[i].y-ps[j].y;
            e[ep++]=(edge){i,j,x*x+y*y};
        }
    }
    f[n]=n;f[n+1]=n+1;
    std::sort(e,e+ep);
    for(int i=0;i<ep;i++){
        
        int a=get(e[i].a),b=get(e[i].b);
        if(a!=b){
            f[a]=b;
            if(get(n)==get(n+1)){
                printf("%.2f\n",sqrt(e[i].c)/2.+1e-8);
                return 0;
            }
        }
    }
    return 0;
}