Traning IOI

vector<pair<int,int>> findTree(vector<int> vec) {
    sort(vec.begin(), vec.end(), [&] (int u, int v) { 
         return sta[u] < sta[v]; 
    });
    int sz = vec.size();
    for(int i = 1; i <= sz - 1; i++) {
        vec.push_back(lca(vec[i - 1], vec[i]));
    }
    sort(vec.begin(), vec.end(), [&] (int u, int v) { 
         return sta[u] < sta[v]; 
    });
    vec.resize(distance(vec.begin(), unique(vec.begin(), vec.end())));
}