Submission #851060

#	Time	Username	Problem	Language	Result	Execution time	Memory
851060		emkow	Network (BOI15_net)	C++17	100 / 100	424 ms	85140 KiB

net

#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
#define pb emplace_back
#define ins insert
#define ssize(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define pii pair <int, int>
#define pll pair <ll, ll>
#define pld pair <ld, ld>
#define st first
#define nd second

using namespace std;
using ll = int_fast64_t;
// using lll = __int128_t;
using ld = long double;
const int oo = 1e9 + 7;
const int mod = 1e9 + 7;

void solve(){
    int n; cin >> n;
    vector <int> deg(n, 0);
    vector <vector<int>> g(n);
    for(int i = 1; i < n; i ++){
        int a, b; cin >> a >> b;
        -- a; -- b;
        g[a].pb(b);
        g[b].pb(a);
        deg[a] ++;
        deg[b] ++;
    }
    vector <int> dp(n);
    int cnt = 0, root;
    for(int i = 0; i < n; i ++){
        if(deg[i] == 1){
            ++ cnt;
            dp[i] = 1;
        }
        else root = i;
    }
    function<void(int, int)> cnt_dp = [&](int v, int p){
        for(auto u: g[v]){
            cnt_dp(u, v);
            dp[v] += dp[u];
        }
    };
    function<int(int, int)> find = [&](int v, int p){
        for(auto u: g[v]){
            if(deg[u] != 1 && dp[u] > 2 * cnt){
                dp[v] -= dp[u];
                dp[u] += dp[v];
                find(u, p);
            }
        }
        return v;
    };
    int cent = find(root, -1);
    vector <int> leaves;
    function<void(int, int)> dfs = [&](int v, int p){
        if(deg[v] == 1) leaves.pb(v);
        for(auto u: g[v]){
            if(u == p) continue;
            dfs(u, v);
        }
    };
    dfs(cent, -1);
    int k = ssize(leaves);
    cout << (k + 1) / 2 << '\n';
    for(int i = 0; i < k / 2; i ++) cout << leaves[i] + 1 << ' ' << leaves[i + k / 2] + 1 << '\n';
    if(k & 1) cout << leaves[0] + 1 << ' ' << leaves.back() + 1 << '\n';
}

signed main(){
    ios_base::sync_with_stdio(0); cin.tie(0);
    int t; t = 1;
    // cin >> t;
    while(t --) solve();
    return 0;
}

Compilation message (stderr)

net.cpp: In function 'void solve()':
net.cpp:34:18: warning: 'root' may be used uninitialized in this function [-Wmaybe-uninitialized]
   34 |     int cnt = 0, root;
      |                  ^~~~

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