Given the root of a binary tree, invert the tree (mirror it), and return its root. Recursive approach: To invert, swap the left and right children at every node, then recursively invert both subtrees. The base case is when the node is null.

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Pythontime O(n) · space O(h) — call stack

def invertTree(root):
    if not root:
        return None
    # Swap children
    root.left, root.right = root.right, root.left
    # Recurse on both subtrees
    invertTree(root.left)
    invertTree(root.right)
    return root

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