Largest Distance between nodes of a Tree

• Time : $O(v + e)$
• Space : $O(v + e)$

int dfs(vector<vector<int>>& adj, int u, int par, int& res) {
    priority_queue<int, vector<int>, greater<int>> q;
    q.push(0);
    q.push(0);
    for(auto& v : adj[u]) {
        if(v == par) continue;
        q.push(dfs(adj,v,u,res));
        if(q.size() > 2) q.pop();
    }
    int less = q.top(); q.pop();
    res = max(res, less + q.top());
    return q.top() + 1;
}
int Solution::solve(vector<int> &A) {
    vector<vector<int>> adj(A.size());
    int root = 0;
    for(int i = 0; i < A.size(); i++) {
        if(A[i] == -1) root = i;
        else {
            adj[A[i]].push_back(i);
            adj[i].push_back(A[i]);
        }
    }
    int res = 0;
    dfs(adj, root, -1, res);
    return res;
}