Submission #79565

#	Time	Username	Problem	Language	Result	Execution time	Memory
79565		EntityIT	Network (BOI15_net)	C++14	0 / 100	12 ms	12148 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;
vector<int> gr[N];
set<ii> ver;

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

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

void dfs (int u, int prev) {
    for (int v : gr[u]) if (v != prev) dfs(v, u);
    if (gr[u].size() == 1) ver.insert(ii(cur, 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);
    }

    for (int u = 1; u <= n; ++u) nLeaf += (gr[u].size() == 1);
    prep(1, 1);
    int cen = findCentroid(1, 1);
    for (int u : gr[cen]) cur ++, dfs(u, cen);

    cout << (nLeaf + 1) / 2 << '\n';
    while (ver.size() > 1) {
        tmp = (*ver.begin() ).se;
        cout << (*ver.begin() ).se << ' ' << (*ver.rbegin() ).se << '\n';
        ver.erase(ver.begin() );
        set<ii>::iterator it = ver.end(); it --;
        ver.erase(it);
    }

    if (ver.size() ) {
        for (int u = 1; u <= n; ++u) if (gr[u].size() == 1 && u != (*ver.begin() ).se) tmp = u;
        cout << tmp << ' ' << (*ver.begin() ).se << '\n';
    }

    return 0;
}

• Subtask 1: 0 / 18
• Subtask 2: 0 / 45
• Subtask 3: 0 / 37