Method add_edge

code/graph/hld.cpp:11–12

                parent(n, -1), loc(n), adj(n) {
    vector<ll> tmp(n, ID); values = segment_tree(tmp); }
  void add_edge(int u, int v) {
    adj[u].push_back(v); adj[v].push_back(u); }
  void update_cost(int u, int v, int c) {
    if (parent[v] == u) swap(u, v); assert(parent[u] == v);
    values.update(loc[u], c); }