Simple binary tree height function.

All I need is a simple function that will return the height of the tree. Can someone please tell me what's wrong with this code...

Code:

---

int bt::tree_height(const node *mytree)
{
  if(mytree == NULL)
  {
  return 0;
  }
  else
  {
    return 1 + maximum(tree_height(mytree -> left), tree_height(mytree -> right));
  }
}

int bt::maximum(int left, int right)
{
  if(left >= right)
    return left;
  else
    return right;
}

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I've done tests and the result is wrong.

Why do you think it is wrong?