树上启发式合并（DSU on tree）

//heavy-light decomposition style .
//http://codeforces.com/blog/entry/44351
int cnt[maxn];
bool big[maxn];
void add(int v, int p, int x){
    cnt[ col[v] ] += x;
    for(auto u: g[v])
        if(u != p && !big[u])
            add(u, v, x)
}
void dfs(int v, int p, bool keep){
    int mx = -, bigChild = -;
    for(auto u : g[v])
       if(u != p && sz[u] > mx)
          mx = sz[u], bigChild = u;
    for(auto u : g[v])
        if(u != p && u != bigChild)
            dfs(u, v, );  // run a dfs on small childs and clear them from cnt
    if(bigChild != -)
        dfs(bigChild, v, ), big[bigChild] = ;  // bigChild marked as big and not cleared from cnt
    add(v, p, );
    //now cnt[c] is the number of vertices in subtree of vertex v that has color c. You can answer the queries easily.
    if(bigChild != -)
        big[bigChild] = ;
    if(keep == )
        add(v, p, -);
}