How to call class from another class in python - python-3.x

Question

I am pretty new and trying to work on an algorithm challenge (Binary Inorder Traversal). I am trying to go through a tree (iterative) but for some reason there is a disconnect when I call the function.
from typing import List
from typing import Optional
# Definition for a binary tree node.
class TreeNode:
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right
class Solution:
def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
res = []
stack = []
cur = root
while cur or stack:
while cur:
stack.append(cur)
cur = cur.left
cur = stack.pop()
res.append(cur.val)
cur = cur.right
print(res)
Solution.inorderTraversal(Solution,[1,'',2,3])
I get "list object has no attribute 'left error and also note that the second value in the root is supposed to be 'null' but when I use 'null' it says null is not defined.

Answer 1

Some things to consider:
Your classes are what you would typically expect in a LeetCode challenge. LeetCode will convert a JSON-like text input for you into a TreeNode instance and will then call your method with that as argument.
The description of such code challenges is most often language agnostic, so there will be mention of null, when in an actual programming language this will be None (Python), NULL (c), ...etc. This is something to take into account.
The function you need to implement is supposed to return the result. Printing it is not enough and not needed
When you want to call your function yourself, you cannot just pass a list as argument, you need to pass a TreeNode instance. For instance, like this:
# This is what LeetCode will do with "[1, null, 2, 3]"
root = TreeNode(1, None, TreeNode(2, TreeNode(3)))
result = Solution().inorderTraversal(root)
print(result)
If you want to dynamically build that TreeNode from a list, you can for instance use a function like provided here.

Answer 2

Here are a few things to take note of, that might help.
First, logically, every instance of your TreeNode class should have as its left and right nodes another instance of the TreeNode class. So to start with, I would change this:
class TreeNode:
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right
...to this:
class TreeNode:
def __init__(self, val: int, left: Optional[TreeNode] = None, right: Optional[TreeNode] = None):
self.val = val
self.left = left
self.right = right
(Note: To use a class's name as a type annotation within the body of that class's definition without triggering a linting error, you may need to place from __future__ import annotations as the very first line of your file. If this is confusing/difficult just drop the type hints.)
Next, to make printing a bit easier on the eye, I suggest adding a __str__() method to your TreeNode class definition, like so:
def __str__(self):
# when printing a TreeNode instance, print its value
return str(self.val)
Next, and most significantly, your Solution class doesn't seem to be necessary. You don't really have a Solution "object" in play here, which rather begs the question - why define it as a class? Also, you have the method returning a list of integers - when in fact what you want it to do (going by the remainder of your method definition) is return a list of TreeNodes.
So your function might look something like this:
def inorderTraversal(root: TreeNode) -> List[TreeNode]:
res = []
cur = root
stack = []
while cur or stack:
while cur:
stack.append(cur)
cur = cur.left
cur = stack.pop(0)
res.append(cur)
cur = cur.right
return res
We can test this on a binary tree composed entirely of TreeNode objects:
root = TreeNode(
val=1,
left=TreeNode(val=2, left=TreeNode(val=4), right=TreeNode(val=5)),
right=TreeNode(val=3, left=TreeNode(val=6), right=TreeNode(val=7)),
)
res = inorderTraversal(root=root)
for node in res:
print(node)
This yields the result:
1
2
4
3
6
5
7