Question

I built a binary tree using a root pointer.

Isn’t the root pointer supposed to be changed in all functions, since I declared it globaly? How do I achieve this?

Thanks

//Pseudocode (left, right pointers declared elsewhere)

Node * root = new Node;


Node * BST::BuildTree(int label)
{
         root->left = changed;
        root->right = changed;
}

Node * BST::GetNode(int label)
{


  BTNode *ptr = root;

  cout << root->right;  //This gives me a seg fault since root is still NULL and not changed
}

Answer 1

This is because you haven’t assign a valid address to root, in your code.

It should point to a valid node:

void BST::CreateRoot()
{
  root = new (std::nothrow) Node;
}

And in C++, you should use unnamed namespace when you want to use a “global variable”. The benefit is that it’ll prevent name conflicts which can easily be introduced by global variables.

namespace
{
  Node * root = NULL;
}

If you’re coding in C, I’d end up here. But since you’re using C++, there’s one thing more.

You should try your best to avoid using global variables, especially when you have lots of them which depend on each other. Usually, you can create a singleton class.

But I don’t think your case is that complicated to use a singleton. Just add an extra input parameter to your functions to indicate which node you want to operate on.

// @param1(node) can be root or whatever node you want
Node * BST::BuildTree(Node *node, int label)
{
  node->left = changed;
  node->right = changed;
}