Largest cycle in a tree

#include <bits/stdc++.h>
using namespace std;

const int N = 100005;

#define ii pair<int, int>
#define fi first
#define sc second

vector<int> a[N];
int vis[N];
int mx = 0, l, r;

ii dfs(int i) {
    vis[i] = 1;
    vector<ii> v;
    for (auto j : a[i]) if (!vis[j]) {
        v.push_back(dfs(j));
    }
    if (v.size() == 0) return ii(0, i);
    else if (v.size() == 1) {
        if (v[0].fi + 1 > mx) {
            mx = v[0].fi + 1;
            l = v[0].sc; r = i;
        }
    }
    else {
        sort(v.begin(), v.end(), greater<ii>());
        if (v[0].fi + v[1].fi + 1 > mx) {
            mx = v[0].fi + v[1].fi + 1;
            l = v[0].sc; r = v[1].sc;
        }
    }
    return ii(v[0].fi + 1, v[0].sc);
}

int main() {
    //freopen("in", "r", stdin);
    int n; cin >> n;
    for (int i = 1; i < n; i++) {
        int u, v; cin >> u >> v;
        a[u].push_back(v);
        a[v].push_back(u);
    }
    l = r = 1; dfs(1);
    cout << l << ' ' << r << '\n';
}