LC1245 AC2

class Solution {
    int ans = 0;
    vector<int> g[10003];
    int dfs(int u, int fa) {
        int a = -1;
        for (auto v: g[u]) {
            if (v == fa) continue;
            int val = dfs(v, u);
            ans = max(ans, a + 1 + val + 1);
            a = max(a, val);
        }
        a += 1;
        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;
    }
};