Submission #96021

#	Time	Username	Problem	Language	Result	Execution time	Memory
96021		popovicirobert	Network (BOI15_net)	C++14	100 / 100	819 ms	73436 KiB

net

#include <bits/stdc++.h>
#define lsb(x) (x & (-x))
#define ll long long
#define ull unsigned long long
#define ld long double
// 217
// 44

using namespace std;

const int MAXN = (int) 5e5;

vector <int> g[MAXN + 1];
int weight[MAXN + 1];

void dfs(int nod, int par) {
    weight[nod] = (g[nod].size() == 1);
    for(auto it : g[nod]) {
        if(it != par) {
            dfs(it, nod);
            weight[nod] += weight[it];
        }
    }
}

int getCentroid(int nod, int par, int cnt) {
    for(auto it : g[nod]) {
        if(it != par && weight[it] > cnt / 2) {
            return getCentroid(it, nod, cnt);
        }
    }
    return nod;
}

vector < vector <int> > leaves;
priority_queue < pair <int, int> > pq;
int id[MAXN + 1];

void dfs1(int nod, int par, int cur) {
    int son = 0;
    for(auto it : g[nod]) {
        if(it != par) {
            dfs1(it, nod, cur);
            son++;
        }
    }
    if(son == 0) {
        leaves[cur].push_back(nod);
        id[nod] = cur;
    }
}

int main() {
    //ifstream cin("A.in");
    //ofstream cout("A.out");
    int i, n;
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    cin >> n;
    for(i = 1; i < n; i++) {
        int x, y;
        cin >> x >> y;
        g[x].push_back(y);
        g[y].push_back(x);
    }
    int root;
    for(i = 1; i <= n; i++) {
        if(g[i].size() > 1) {
            root = i;
            break;
        }
    }
    dfs(root, 0);
    root = getCentroid(root, 0, weight[root]);
    int sz = g[root].size();
    leaves.resize(sz);
    for(i = 0; i < sz; i++) {
        dfs1(g[root][i], root, i);
    }
    for(i = 0; i < sz; i++) {
        pq.push({leaves[i].size(), i});
    }
    vector < pair <int, int> > sol;
    int leaf = 0;
    while(pq.size() > 1) {
        int id1 = pq.top().second;
        pq.pop();
        int id2 = pq.top().second;
        pq.pop();
        sol.push_back({leaves[id1].back(), leaves[id2].back()});
        if(leaf == 0) {
            leaf = leaves[id1].back();
        }
        leaves[id1].pop_back();
        leaves[id2].pop_back();
        if(leaves[id1].size()) {
            pq.push({leaves[id1].size(), id1});
        }
        if(leaves[id2].size()) {
            pq.push({leaves[id2].size(), id2});
        }
    }
    if(pq.size()) {
        sol.push_back({leaf, leaves[pq.top().second].back()});
    }
    cout << sol.size() << "\n";
    for(auto it : sol) {
        cout << it.first << " " << it.second << "\n";
    }
    //cin.close();
    //cout.close();
    return 0;
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:74:10: warning: 'root' may be used uninitialized in this function [-Wmaybe-uninitialized]
     root = getCentroid(root, 0, weight[root]);
     ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• Subtask 1: 18 / 18
• Subtask 2: 45 / 45
• Subtask 3: 37 / 37