I am new to navigating Binary Tree's. I was able to eventually figure out a solution to this problem looking through some of the answers in the discussion.

I am still not sure why I was not able get this code to work. I think there is something about the nodes that I do not understand. They are not iterable?

I also attempted to work throught the solution by using a hash map then a try, except and appending to a list that would return the answer.

Please let me know what I am missing.

class Solution:
    def findDuplicateSubtrees(self, root: TreeNode) -> List[TreeNode]:
        if not root:
            return []
        
        a = []
        b = []
        def fr(root):
            if not root: return
            if root in a:
                b.append(root)
            else:
                a.append(root)
            fr(root.left)
            fr(root.right)
        fr(root)
        
        
        return b