Submission #1033251

#	Time	Username	Problem	Language	Result	Execution time	Memory
1033251		aufan	Network (BOI15_net)	C++17	100 / 100	379 ms	69676 KiB

net

#include <bits/stdc++.h>
#define int long long
#define fi first
#define se second

using namespace std;

int32_t main()
{
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        
        int n;
        cin >> n;

        vector<vector<int>> v(n + 1);
        for (int i = 0; i < n - 1; i++) {
                int a, b;
                cin >> a >> b;

                v[a].push_back(b);
                v[b].push_back(a);
        }
        
        vector<int> dp(n + 1);

        function<void(int, int)> dfs = [&](int x, int p) {
                if ((int)v[x].size() == 1) {
                        dp[x] += 1;
                }

                for (auto z : v[x]) {
                        if (z == p) continue;

                        dfs(z, x);

                        dp[x] += dp[z];
                }
        };
        
        int tot;
        for (int i = 1; i <= n; i++) {
                if ((int)v[i].size() > 1) {
                        dfs(i, i);
                        tot = dp[i];
                        break;
                }
        }

        int mn = n + 1, rt = -1;
        for (int i = 1; i <= n; i++) {
                if (dp[i] >= tot / 2) {
                        if (dp[i] < mn) {
                                mn = dp[i];
                                rt = i;
                        }
                }
        }

        vector<int> leafs;

        function<void(int, int)> dfs2 = [&](int x, int p) {
                if ((int)v[x].size() == 1) {
                        leafs.push_back(x);
                }

                for (auto z : v[x]) {
                        if (z == p) continue;

                        dfs2(z, x);
                }
        };

        dfs2(rt, rt);

        if ((int)leafs.size() % 2 == 1) {
                leafs.push_back(leafs.back());
        }

        cout << (int)leafs.size() / 2 << '\n';
        for (int i = 0; i < (int)leafs.size() / 2; i++) cout << leafs[i] << " " << leafs[i + (int)leafs.size() / 2] << '\n';
        
        return 0;
}

Compilation message (stderr)

net.cpp: In function 'int32_t main()':
net.cpp:52:34: warning: 'tot' may be used uninitialized in this function [-Wmaybe-uninitialized]
   52 |                 if (dp[i] >= tot / 2) {
      |                              ~~~~^~~

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