LC1245 AC

class Solution {
    int ans = 0;
    vector<int> g[10003];
    int dfs(int u, int fa) {
        int a = -1, b = -1;
        for (auto v: g[u]) {
            if (v == fa) continue;
            int val = dfs(v, u);
            if (val > a) b = a, a = val;
            else if (val > b) b = val;
        }
        a += 1;
        b += 1;
        // cout << "u: " << u << " a: " << a << " b: " << b << endl;
        ans = max(ans, a + b);
        return a;
    }
public:
    int treeDiameter(vector<vector<int>>& edges) {
        for (const auto& e: edges) g[e[0]].push_back(e[1]), g[e[1]].push_back(e[0]);
        dfs(0, -1);
        return ans;
    }
};