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| #include<algorithm>
#include<iostream>
#include<iomanip>
#include<cstring>
#include<cstdlib>
#include<vector>
#include<cstdio>
#include<cmath>
#include<queue>
using namespace std;
typedef long long LL;
inline const int Get_Int() {
int num=0,bj=1;
char x=getchar();
while(x<'0'||x>'9') {
if(x=='-')bj=-1;
x=getchar();
}
while(x>='0'&&x<='9') {
num=num*10+x-'0';
x=getchar();
}
return num*bj;
}
const int maxn=6005;
struct Edge {
int from,to;
LL dist;
};
vector<Edge>edges[maxn];
void AddEdge(int x,int y,LL v) {
edges[x].push_back((Edge) {
x,y,v
});
}
int n,m,k,vst[maxn],From[maxn];
LL dist[maxn],x[maxn],y[maxn],ans=0;
LL Dist(int a,int b) {
if(a>k||b>k)return (y[a]-y[b])*(y[a]-y[b]);
return (x[a]-x[b])*(x[a]-x[b])+(y[a]-y[b])*(y[a]-y[b]);
}
void Prim() {
for(int i=1; i<=k+2; i++)dist[i]=1e17;
dist[k+1]=0;
for(int i=1; i<=k+2; i++) {
LL Min=1e17;
int id=0;
for(int j=1; j<=k+2; j++)
if(!vst[j]&&dist[j]<Min) {
Min=dist[j];
id=j;
}
vst[id]=1;
if(From[id]) {
LL d=Dist(id,From[id]);
AddEdge(id,From[id],d);
AddEdge(From[id],id,d);
}
for(int j=1; j<=k+2; j++) {
LL d=Dist(id,j);
if(!vst[j]&&d<dist[j]) {
dist[j]=d;
From[j]=id;
}
}
}
}
void Dfs(int Now,int father,LL Max) {
if(Now==k+2) {
ans=Max;
return;
}
for(auto& e:edges[Now]) {
int Next=e.to;
if(Next==father)continue;
Dfs(Next,Now,max(Max,e.dist));
}
}
int main() {
n=Get_Int();
m=Get_Int();
k=Get_Int();
for(int i=1; i<=k; i++) {
x[i]=Get_Int();
y[i]=Get_Int();
}
y[k+2]=m;
Prim();
Dfs(k+1,0,0);
printf("%0.8lf\n",sqrt(ans)/2);
return 0;
}
|
