POI Task Parade

#include <bits/stdc++.h>

using namespace std;

const int INF = 1e9;

pair<int, int> dfs(int v, const vector<vector<int>> &g, int parent = 0) {
    int maxFC = -INF, maxEGU = -INF, maxEGU2 = -INF;
    for (int u : g[v]) {
        if (u != parent) {
            auto [fc, egu] = dfs(u, g, v);
            maxFC = max(maxFC, fc);
            if (egu > maxEGU2) {
                maxEGU2 = egu;
                if (maxEGU2 > maxEGU) swap(maxEGU, maxEGU2);
            }
        }
    }

    int degreeIncludingParent = (int)g[v].size();
    int degreeExcludingParent = (int)g[v].size() - !!parent;
    int fullyContained = max({maxFC, degreeIncludingParent-1+maxEGU, degreeIncludingParent-2+maxEGU+maxEGU2});
    int edgeGoingUp = max({degreeExcludingParent, degreeExcludingParent-1+maxEGU});
    return {fullyContained, edgeGoingUp};
}

int main() {
    int n; cin >> n;
    vector<vector<int>> g(1+n);
    for (int i = 1; i < n; ++i) {
        int u, v; cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    cout << dfs(1, g).first << endl;
    return 0;
}