Submission #1250856

#	Time	Username	Problem	Language	Result	Execution time	Memory
1250856		tradz	Network (BOI15_net)	C++20	0 / 100	12 ms	23876 KiB

net

#include <bits/stdc++.h>
#define TIME  (1.0 * clock() / CLOCKS_PER_SEC)
#define For(i,a,b) for(int i = a; i <= b; i++)
#define Ford(i,a,b) for(int i = a; i >= b; i--)
#define ll long long
#define ii pair<int,int>
#define fi first
#define se second
#define all(v) v.begin(),v.end()
#define RRH(v) v.resize(unique(all(v)) - v.begin())

using namespace std;
const int  N = 1e6+7;
const int M = 1e9+7;
const ll oo = 3e18;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

long long GetRandom(long long l, long long r) {
    return uniform_int_distribution<long long> (l, r)(rng);
}

int n, cnt = 0;
vector <int> g[N];

int timedfs = 0, tin[N], tout[N];

int dp[N];

void dfs(int u, int pre = 0) {
    tin[u] = tout[u] = ++timedfs;
    dp[u] = 1;
    for (int v: g[u]) {
        if (v == pre) continue;
        dfs(v, u);
        dp[u] += dp[v];
    }
    tout[u] = timedfs;
}

int main() {
    ios::sync_with_stdio(0); cin.tie(0);

    #define TASK ""
    if (fopen (".inp", "r")) {
        freopen (".inp", "r", stdin);
        freopen (".out", "w", stdout);
    }
    if(fopen(TASK".inp", "r")) {
        freopen(TASK".inp", "r", stdin);
        freopen(TASK".out", "w", stdout);
    }

    cin >> n;

    for (int i = 1; i < n; i++) {
        int u, v; cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
    }



    vector <pair <int, int>> ans;

    for (int i = 1; i <= n; i++) {
        if (g[i].size() >= 2) {
            dfs(i, 1);
            break;
        }
    }
    vector <pair <int, int>> ver;
    for (int i = 1; i <= n; i++) {
        if (dp[i] == 1) {
            ver.push_back({tin[i], i});
        }
    }

    sort (all(ver));

    for (int i = 0; i < ver.size(); i += 2) {
        int a = i;
        int b = (i + 1) % ver.size();
        ans.push_back({ver[a].se, ver[b].se});
    }


    cout << ans.size() << '\n';
    for (int i = 0; i < ans.size(); i++) cout << ans[i].first << " " << ans[i].second << '\n';

    cerr << "Time elapsed: " << TIME << " s.\n";
    return 0;
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:46:17: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   46 |         freopen (".inp", "r", stdin);
      |         ~~~~~~~~^~~~~~~~~~~~~~~~~~~~
net.cpp:47:17: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   47 |         freopen (".out", "w", stdout);
      |         ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
net.cpp:50:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   50 |         freopen(TASK".inp", "r", stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
net.cpp:51:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   51 |         freopen(TASK".out", "w", stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

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