0226. Invert Binary Tree

https://leetcode.com/problems/invert-binary-tree

Description

Given the root of a binary tree, invert the tree, and return its root.

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:** []

Constraints:

• The number of nodes in the tree is in the range [0, 100].

• -100 <= Node.val <= 100

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