void levelOrder(Node * root){
 vector<Node*> oQ;
 
 if (nullptr != root) {
  oQ.push_back(root);

  for(int i=0; i<oQ.size(); i++) {
   cout<<oQ[i]->data<<" ";

   if (nullptr != oQ[i]->left)
    oQ.push_back(oQ[i]->left);

   if (nullptr != oQ[i]->right)
    oQ.push_back(oQ[i]->right);
  }
 }
}