Method update_cost

code/graph/hld.cpp:13–15

  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); }
  int csz(int u) {
    rep(i,0,size(adj[u])) if (adj[u][i] != parent[u])
      sz[u] += csz(adj[parent[adj[u][i]] = u][i]);