Submission #1240727

#	Time	Username	Problem	Language	Result	Execution time	Memory
1240727		Zero	Mobile (BOI12_mobile)	C++20	0 / 100	1099 ms	196608 KiB

mobile

#include <bits/stdc++.h>
#define f first
#define ss second
using namespace std;

bool ok(double r, const vector<pair<int, int>>& a, int x_target) {
    int n = a.size();
    vector<vector<int>> g(n);
    
    // Build graph
    for(int i = 0; i < n; ++i){
        for(int j = i+1; j < n; ++j){
            double dx = a[i].f - a[j].f;
            double dy = a[i].ss - a[j].ss;
            if(dx*dx + dy*dy <= 4*r*r){
                g[i].push_back(j);
                g[j].push_back(i);
            }
        }
    }

    // Find which phones can reach x=0 and which can reach x=x_target
    vector<int> starts, targets;
    for(int i = 0; i < n; ++i){
        if(a[i].f - r <= 0) starts.push_back(i);
        if(a[i].f + r >= x_target) targets.push_back(i);
    }

    // Try to reach from any start to any target
    queue<int> q;
    vector<bool> visited(n, false);
    for(int s : starts){
        q.push(s);
        visited[s] = true;
    }

    while(!q.empty()){
        int u = q.front(); q.pop();
        for(int v : g[u]){
            if(!visited[v]){
                visited[v] = true;
                q.push(v);
            }
        }
    }

    for(int t : targets){
        if(visited[t]) return true;
    }

    return false;
}


signed main(){
    cin.tie(0)->sync_with_stdio(0);
    int n,x; cin >> n >> x;
    vector<pair<int,int>> a(n);
    for(auto &i : a) cin >> i.first >> i.second;
    
    double l = 0, r = INT_MAX;
    for(int i=0; i < 100; i ++){
        double m = (l + r) / 2;
        if(ok(m,a,x)) r = m;
        else l=m;
    }
    cout << fixed << setprecision(6) << r;
}

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