Submission #1097709

#	Time	Username	Problem	Language	Result	Execution time	Memory
1097709		TrinhKhanhDung	Network (BOI15_net)	C++14	100 / 100	264 ms	47952 KiB

net

#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define sz(x) (int)x.size()
#define ALL(v) v.begin(),v.end()
#define MASK(k) (1LL << (k))
#define BIT(x, i) (((x) >> (i)) & 1)
#define oo (ll)1e18
#define INF (ll)1e9
#define MOD (ll)(1e9 + 7)

using namespace std;

template<class T1, class T2>
    bool maximize(T1 &a, T2 b){if(a < b){a = b; return true;} return false;}

template<class T1, class T2>
    bool minimize(T1 &a, T2 b){if(a > b){a = b; return true;} return false;}

template<class T1, class T2>
    void add(T1 &a, T2 b){a += b; if(a >= MOD) a -= MOD;}

template<class T1, class T2>
    void sub(T1 &a, T2 b){a -= b; if(a < 0) a += MOD;}

template<class T>
    void cps(T &v){sort(ALL(v)); v.resize(unique(ALL(v)) - v.begin());}

const int MAX = 5e5 + 10;

int N;
vector<int> adj[MAX];
vector<int> leaves;

void dfs(int u, int p = -1){
    if(sz(adj[u]) == 1) leaves.push_back(u);
    for(int v: adj[u]){
        if(v == p) continue;
        dfs(v, u);
    }
}

void solve(){
    cin >> N;
    for(int i = 1; i < N; i++){
        int u, v; cin >> u >> v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }

    clock_t start = clock();

    if(N == 2){
        cout << 1 << '\n' << "1 2\n";
        return;
    }

    int root;
    for(int i = 1; i <= N; i++){
        if(sz(adj[i]) > 1){
            root = i;
            break;
        }
    }

    dfs(root);

    vector<pair<int, int>> ans;
    int n = sz(leaves);
    for(int i = 0; i < n / 2; i++){
        ans.push_back({leaves[i], leaves[i + n / 2]});
    }

    if(sz(leaves) & 1){
        ans.push_back({leaves[0], leaves.back()});
    }

    cout << sz(ans) << '\n';
    for(auto o: ans){
        cout << o.fi << ' ' << o.se << '\n';
    }

    cerr << "Time elapsed: " << clock() - start << " ms\n";
}

int main(){
    ios_base::sync_with_stdio(0); cin.tie(0);
//    freopen("walk.inp","r",stdin);
//    freopen("walk.out","w",stdout);

    int t = 1;
//    cin >> t;
    while(t--)
        solve();

    return 0;
}

Compilation message (stderr)

net.cpp: In function 'void solve()':
net.cpp:67:8: warning: 'root' may be used uninitialized in this function [-Wmaybe-uninitialized]
   67 |     dfs(root);
      |     ~~~^~~~~~

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