如何使用Stringstream C ++递归打印二进制搜索树中的所有节点

Question

I would like to print the contents of all the nodes in my binary search tree by using a stringstream and recursion. The issue is that when I use this code, only the contents of the root are displayed. I know the reason is that every time I recursively call the function InOrder(BSTNode* bst_node), my stringstream variable is created again. What can I do to my code to fix this issue while still utilizing a stringstream for output?

This is my code:

string BSTree::InOrder(BSTNode* bst_node) {
  stringstream ss;
  if (root_ ==  NULL) {
    return "";
  } else {
    if (bst_node->GetLeftChild() != NULL) {
      InOrder(bst_node->GetLeftChild());
    }
    ss << bst_node->GetContents() << " ";
    if (bst_node->GetRightChild() != NULL) {
      InOrder(bst_node->GetRightChild());
    }
  }
  return ss.str();
}

Answer 1

Something like that maybe?

string BSTree::InOrder(BSTNode* bst_node)
{
  if (!bst_node)
    return "";
  ostringstream ss;
  ss << InOrder(bst_node->GetLeftChild());
  ss << bst_node->GetContents() << " ";
  ss << InOrder(bst_node->GetRightChild());
  return ss.str();
}

or, you can pass around the same instance of stringstream:

void BSTree::InOrderImpl(BSTNode* bst_node, ostringstream &ss)
{
  if (bst_node)
  {
    InOrderImpl(bst_node->GetLeftChild(), ss);
    ss << bst_node->GetContents() << " ";
    InOrderImpl(bst_node->GetRightChild(), ss);
  }
}

string BSTree::InOrder(BSTNode* bst_node)
{
  ostringstream ss;
  InOrder(bst_node, ss);
  return ss.str();
}

Answer 2

Easy: just define an utility function where stringstream is passed as argument:

void BSTree::InOrder(BSTNode *root, std::stringstream &ss)
{
  if (root == nullptr) { return; }
  InOrder(root->GetLeftChild(), ss)
  ss << root->GetContents() << " ";
  InOrder(root->GetRightChild(), ss)
}

string BSTree::InOrder(BSTNode* bst_node) {
  stringstream ss;
  InOrder(bst_node, ss);
  return ss.str();
}

Notice that instantiating only one stringstream where the whole tree is dumped is probably much more efficient than instantiating a stringstream for each node of your tree (as it happens if you define the utility function as something taking a node and returning a string).

Answer 3

My no brainer for this - pass the stringstream as argument by reference to this method:

static stringstream existingSSreference;
string BSTree::InOrder(BSTNode* bst_node, stringstream & ss = existingSSreference) {

  if (root_ ==  NULL) {
    return "";
  } else {
    if (bst_node->GetLeftChild() != NULL) {
      InOrder(bst_node->GetLeftChild(), ss);
    }
    ss << bst_node->GetContents();
    if (bst_node->GetRightChild() != NULL) {
      InOrder(bst_node->GetRightChild(), ss);
    }
  }
  return ss.str();
}

and either you want to declare the stringstream before using the method:

stringstream ss;
string myResult = bstreeObj->InOrder(bstNode, ss);

or use it without passing it

string myResult = bstreeObj->InOrder(bstNode);

this should work. Better define the stringstream outside the function and pass it when you call the function.

EDIT

As pointed out by Ajay, a reference argument can't have default value, unless we pass an actual existing instance as default (so that we can keep the possibility to omit the second argument).