Submission #79592

#	Time	Username	Problem	Language	Result	Execution time	Memory
79592		EntityIT	Network (BOI15_net)	C++14	100 / 100	977 ms	80828 KiB

net

#include<bits/stdc++.h>

using namespace std;

#define pb push_back
#define fi first
#define se second
typedef pair<int, int> ii;

const int N = 5e5 + 5;
int n, child[N], nLeaf, cur, tmp, pter[N];
vector<int> gr[N], vec[N];

void prep (int u, int p) {
    for (int v : gr[u]) if (v != p) {
        prep(v, u);
        child[u] += child[v];
    }
    child[u] += (gr[u].size() == 1);
}

int findCentroid (int u, int p) {
    for (int v : gr[u]) if (v != p) {
        if (child[v] >= (nLeaf + 1) / 2 ) return findCentroid(v, u);
    }
    return u;
}

void dfs (int u, int p) {
    for (int v : gr[u]) if (v != p) dfs(v, u);
    if (gr[u].size() == 1) vec[cur].pb(u);
}

int main () {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    cin >> n;
    for (int _ = 1; _ < n; ++_) {
        int u, v; cin >> u >> v;
        gr[u].pb(v); gr[v].pb(u);
    }
    int root;
    for (int i = 1; i <= n; ++i) if (gr[i].size() ) root = i;

    for (int u = 1; u <= n; ++u) nLeaf += (gr[u].size() == 1);
    cout << (nLeaf + 1) / 2 << '\n';
    prep(root, root);
    int cen = findCentroid(root, root);

    for (int u : gr[cen]) cur ++, dfs(u, cen);

    priority_queue<ii> pq;
    for (int i = 1; i <= cur; ++i) {
        pq.push(ii(vec[i].size() - pter[i], i) );
    }

    while (pq.size() > 1) {
        ii tmp1 = pq.top(); pq.pop();
        ii tmp2 = pq.top(); pq.pop();

        cout << vec[tmp1.se][ pter[tmp1.se] ] << ' ' << vec[tmp2.se][ pter[tmp2.se] ] << '\n';
        pter[tmp1.se] ++; pter[tmp2.se] ++;
        if (pter[tmp1.se] < vec[tmp1.se].size() ) pq.push(ii(vec[tmp1.se].size() - pter[tmp1.se], tmp1.se) );
        if (pter[tmp2.se] < vec[tmp2.se].size() ) pq.push(ii(vec[tmp2.se].size() - pter[tmp2.se], tmp2.se) );
    }

    if (pq.size() ) {
        int u = vec[pq.top().se][ pter[pq.top().se] ];
        for (int v = 1; v <= n; ++v) if (gr[v].size() == 1 && v != u) { cout << v << ' ' << u << '\n'; break; }
    }

    return 0;
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:62:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if (pter[tmp1.se] < vec[tmp1.se].size() ) pq.push(ii(vec[tmp1.se].size() - pter[tmp1.se], tmp1.se) );
             ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
net.cpp:63:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if (pter[tmp2.se] < vec[tmp2.se].size() ) pq.push(ii(vec[tmp2.se].size() - pter[tmp2.se], tmp2.se) );
             ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
net.cpp:47:38: warning: 'root' may be used uninitialized in this function [-Wmaybe-uninitialized]
     int cen = findCentroid(root, root);
                                      ^

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