Invert Binary Tree (LeetCode #226)

Challenge Statement

• Given the root of a binary tree, invert the tree, and return its root.
• This challenge corresponds to LeetCode #226.

Constraints

• The number of nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

Example 1:

Input: root = [4, 2, 7, 1, 3, 6, 9]
Output: [4, 7, 2, 9, 6, 3, 1]

Example 2:

Input: root = [2, 1, 3]
Output: [2, 3, 1]

Example 3:

Input: root = []
Output: []

Solution

Below is my solution and some test cases. This solution has a linear time complexity O(n) and a constant space complexity O(1), where n is the size of the tree.