Invert Binary Tree 15 XP Easy

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Invert a binary tree — swap every left and right child. This is the famous problem that "Homebrew's creator Max Howell couldn't solve at Google."

# Before:        4            After:        4
#              /   \                       /   \
#             2     7         →           7     2
#            / \   / \                   / \   / \
#           1   3 6   9                 9   6 3   1

The simplest recursive solution:

Base case: If node is None, return None
Swap: Switch left and right children
Recurse: Invert both subtrees
Time: O(n), Space: O(h)

This is a great example of how elegant recursion can be — just 3 lines of code!

📋 Instructions

Implement `invert_tree(root)` and verify by doing an inorder traversal before and after inversion.

Recursion! Base case: if root is None, return None. Swap left and right children: root.left, root.right = root.right, root.left. Then recurse on both.

🧪 Test Cases

Input

inorder(root)

Expected

[1, 2, 3, 4, 6, 7, 9]

Test case 1

Input

inorder(root)

Expected

[9, 7, 6, 4, 3, 2, 1]

Test case 2

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main.py

Hi! I'm Rex 👋

Output

Ready. Press ▶ Run or Ctrl+Enter.

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