Error while counting nodes of binary search tree

Question

I'm trying to write a function that counts the number of nodes of a binary search tree. However, I'm getting an error when I execute that function. This is the error: 'NoneType' object has no attribute 'right'. The error is for the line p=p.right.

Let me know if more data is to be provided.

Here is the function:

def countelements(self, root):
    stack=[]
    p=root
    count=0
    while p!=None:
        stack.append(p)
        p=p.left
        count=count+1
    while (len(stack)>0):
        p=stack.pop()
        p=p.right
        while p!=None:
            p=p.left
            stack.append(p)
            count=count+1
            
    return count

Answer 1

I haven't thought through your core logic, but the source of the problem of why there is a None inside your stack must be the inner loop (See inline comment below).

        while p!=None:
            p=p.left # But what if p.left is None?
            stack.append(p)
            count=count+1

Answer 2

When you work with recursive data structures, such as binary tree, you often want to use recursion.

def count_elements(root):
    if root is not None:
        left = count_elements(root.left)
        right = count_elements(root.right)
        return left + right + 1  # +1 for root itself
    else:
        return 0