void SeparateVertices(vector<VertexId>& output, size_t size) {
TreeNode<VertexId>* node = root_.GetSubtreeWithSize(min(size, GetSize()));
if (node == 0) {
node = &root_;
}
output.reserve(node->GetSize());
CollectValues(node, output);
}
TreeNode* GetSubtreeWithSize(const size_t size) {
// find big enough child.
auto it = std::find_if(children_.begin(), children_.end(), SizePred(size));
if (it == children_.end()) {
return 0;
}
TreeNode* node = (*it)->GetSubtreeWithSize(size);
if (node == 0) {
node = *it;
children_.erase(it);
}
subtree_size_ -= node->GetSize();
return node;
}
