§ Disjoint set union


§ intuition for correctness of rank:


Assume that we had to re-point pointers of all our children to the new root when we decide to make another node the root. That is, we would have:
void mkroot(int newroot, int prevroot) {
   for (int child : children[prevroot] {
        parent[child] = newroot;
   }
   parent[prevroot] = newroot];
   children[prevroot] = {}; // this has no children anymore
}