text: //Function to return a list containing the preorder traversal of the tree.
  void PREORDER(Node* root,vector <int>&v)
  {
      if(root == NULL)
          return;
      v.push_back(root->data);
      PREORDER(root->left,v);
      PREORDER(root->right,v);
      
  }
  
  vector <int> preorder(Node* root)
  {
    vector <int> v;
    PREORDER(root,v);
    return v;
  }