P1783 海滩防御

jisoo

简单的二分

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<cstring>
using namespace std;
int n,m;
struct po{
	int x;
	int y;
}p[1005];
int x,y;
int r[1005];
bool cmp(po x,po j){
	return x.x<j.x;
}
double l,rr,mid;
double ans;
double dis[1005][1005];
int vis[1005];
bool dfs(int x,double mid){
	//cout<<x<<endl;
	if((n-p[x].x)*2<=mid){
		return 1;
	}else{
		for(int i=1;i<=m;++i){
			if(dis[x][i]<=mid&&i!=x&&!vis[i]){
				vis[i]=1;
				if(dfs(i,mid)) return 1;
			}
		}
		return 0;
	}
	return 0;
}
bool check(double aim){
	if(aim>=p[1].x&&aim>=(n-p[m].x)){
		for(int i=1;i<=m;++i){
			if(p[i].x>aim)
			break;
			if(vis[i]) continue;
			vis[i]=1;
			if(dfs(i,aim*2))
				return 1;
		}
	}
	return 0;
}
int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;++i){
		scanf("%d%d",&p[i].x,&p[i].y); 
	}
	rr=n;
//	for(int i=1;i<=m;++i){
//		cout<<r[i]<<endl;
//	}
	sort(p+1,p+m+1,cmp);
	for(int i=1;i<=m;++i){
		for(int j=i+1;j<=m;++j){
			double di=(double)sqrt((p[i].x-p[j].x)*(p[i].x-p[j].x)+(p[i].y-p[j].y)*(p[i].y-p[j].y));
			dis[i][j]=dis[j][i]=di;
		}
	}
	while(l+1e-6<=rr){
		mid=(l+rr)/2; 
		if(check(mid)){
			rr=mid;
			ans=mid;
		} else{
			l=mid;
		}
		memset(vis,0,sizeof(vis));
	}
	printf("%.2lf",ans);
	return 0;
}