Merge two Binary Search Trees (BST)

void merge_bst(node* root1, node* root2)
  {
  stack<node * > s1;
  stack<node * > s2;
  vector<int> res;
  node * temp1= root1;
  node * temp2= root2;
  while(temp1!=NULL){
  s1.push(temp1);
  temp1= temp1->left;
  }
  while(temp2!=NULL)
  {
  s2.push(temp2);
  temp2= temp2->left;
  }
  while(!s1.empty()&& ! s2.empty())
  {
    if(s1.top()->data<= s2.top()->data)
    {
    res.push_back(s1.top()->data);
    temp1= s1.top()->right;
    s1.pop();
    while(temp1!=NULL)
    {
    s1.push(temp1);
    temp1= temp1->left;
    }
    }
    else
    {
    res.push_back(s2.top()->data);
    temp1= s2.top()->right;
    s2.pop();
    while(temp1!=NULL)
    {
    s2.push(temp1);
    temp1= temp1->left;
    }
    }
  }
  while(!s1.empty())
  {
    res.push_back(s1.top()->data);
    temp1= a.top()->right;
    s1.pop();
    while(temp1!=NULL)
    {
    s1.push(temp1);
    temp1= temp1->left;
    }
  }
  while(!s2.empty())
  {
    res.push_back(b.top()->data);
    temp1= s2.top()->right;
    s2.pop();
    while(temp1!=NULL)
    {
    s2.push(temp1);
    temp1 = temp1->left;
    }
  }
  print(res);
}