Inorder Binary Tree Traversal (using Python)

Question

I am trying to perform an inorder traversal of a tree. The code itself feels right, except it is not working properly. I have a feeling it has to either do with the if condition, how append works in python, or something perhaps with return. This works correctly if I use print instead of return, I think, but I want to be able to use return and still get the correct answer. For example, for the tree [1,None,2,3], my code returns [1] which is clearly incorrect.

Additionally is it possible to solve this problem using list comprehension? If so, any sample code would be greatly appreciated.

Here is my code:

    class Solution(object):
        def inorderTraversal(self, root):
            res = []
            if root:
                self.inorderTraversal(root.left)
                res.append(root.val)
                self.inorderTraversal(root.right)
            return res

Also before marking this as a duplicate, I know in order traversals have been asked on Stackoverflow (plenty of times), but none of them helped me understand why my understanding is wrong. I would be so grateful if someone helped me learn how to correct my approach versus simply posting another link without explanation. Thank you so much!

Answer 1

The reason this doesn't work is that res only has the value of the first node you give it appended to it; each time you recursively recall the function, it just makes a new res. It is a simple fix though, as follows:

class Solution(object):
    def inorderTraversal(self, root):
        res = []
        if root:
            res = self.inorderTraversal(root.left) 
            res.append(root.val)
            res = res + self.inorderTraversal(root.right)
        return res

In this, it returns the left branch, the value, and then the right. This can be done much more briefly as follows:

class Solution(object):
    def inorderTraversal(self, root):
        return (self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)) if root else []