Submission #161420

#	Time	Username	Problem	Language	Result	Execution time	Memory
161420		Minnakhmetov	Mousetrap (CEOI17_mousetrap)	C++14	100 / 100	1319 ms	219628 KiB

mousetrap

#include <bits/stdc++.h>
    
#define ll long long
#define all(aaa) aaa.begin(), aaa.end()
 
using namespace std;
 
const int N = 1e6 + 5, INF = 1e9;
vector<int> g[N];
int n, m, t;
bool term[N];

vector<vector<int>> w;

void dive(int node, int anc) {
    if (node == t)
        term[node] = 1;
    for (int to : g[node]) {
        if (to != anc) {
            dive(to, node);
            if (term[to])
                term[node] = 1;
        }
    }
}
 
int dfs(int node, int anc) {
    int adj = g[node].size() - (anc != -1);
 
    vector<int> v;
 
    for (int to : g[node]) {
        if (to != anc) {
            v.push_back(dfs(to, node) + 1);
        }
    }
 
    sort(v.rbegin(), v.rend());
 
    if (v.empty())
        return 0;
    else if (v.size() == 1)
        return 1;
    return v[1] + adj - 1;
}

int walk(int node, int anc) {
    if (node == t)
        return 0;

    vector<int> v;

    for (int to : g[node]) {
        if (to != anc && !term[to]) {
            v.push_back(dfs(to, node) + 1);                
        }
    }

    int add = 0;

    for (int to : g[node]) {
        if (to != anc) {
            if (term[to]) {
                add = walk(to, node);
            }   
        }
    }

    for (int &x : v)
        x += v.size() - 1 + add;

    sort(v.begin(), v.end());

    w.push_back(v);

    add += v.size();

    return add;
}
 
signed main() {
    ios_base::sync_with_stdio(0);
    cin.tie(NULL);
 
    cin >> n >> t >> m;
 
    m--, t--;
 
    for (int i = 0; i < n - 1; i++) {
        int a, b;
        cin >> a >> b;
        a--, b--;
        g[a].push_back(b);
        g[b].push_back(a);
    }

    dive(m, -1);
    walk(m, -1);


    reverse(all(w));

    // for (auto &v : w) {
    //     for (int x : v) {
    //         cout << x << " ";
    //     }
    //     cout << "\n";
    // }

    int l = -1, r = INF;
    while (r - l > 1) {
        int p = (l + r) >> 1;

        bool ok = true;
        int energy = 0, expended = 0;
        for (auto &v : w) {
            energy++;

            for (int x : v) {
                if (x + expended > p) {
                    if (energy == 0 || expended + 1 > p) {
                        ok = false;
                        break;
                    }
                    expended++;
                    energy--;
                }
            }
            if (!ok)
                break;
        }

        if (ok)
            r = p;
        else
            l = p;
    }

    cout << r << "\n";
 
    return 0;
}

• Subtask 1: 20 / 20
• Subtask 2: 25 / 25
• Subtask 3: 20 / 20
• Subtask 4: 35 / 35