Invert a Binary Tree — Algorithm interview question

Given a binary tree, explain how you would invert it (mirror it). Every left child becomes the right child and vice versa, at every level.

For example:

    Input:        Output:
      4             4
     / \           / \
    2   7         7   2
   / \ / \       / \ / \
  1  3 6  9     9  6 3  1

How do you ensure every node in the tree gets swapped? What are the edge cases?

Explain your approach and analyze the time and space complexity.

How to approach it

Hint 1
At each node, you need to swap its left and right children.
Hint 2
After swapping the children of the current node, you need to recursively invert the left and right subtrees.
Hint 3
Base case: if the node is null, return null. A single-node tree is already its own mirror.

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