Inorder Binary Tree Traversal (using Python)

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