Invert Binary Tree

You get the root of a binary tree. Turn it into its mirror image and return the root.

In the mirror image, every node's left and right children swap places. This is called inverting the tree.

The trees below are listed level by level, top to bottom and left to right. A missing child is written as null.

The test results print a tree in the same order, with spaces between the values. A missing child prints as -1, and the -1s at the end are left out. For example, [6,9,2,null,7,4] prints as 6 9 2 -1 7 4. An empty tree prints as -1.

Example 1:

       6                     6
     /   \                 /   \
    2     9       →       9     2
     \   /                 \   /
      4 7                   7 4

Input: root = [6,2,9,null,4,7]
Output: [6,9,2,null,7,4]
Explanation: 2 and 9 swap places under the root.
4 moves from the right of 2 to its left.
7 moves from the left of 9 to its right.

Example 2:

  8                    8
   \                  /
    5       →        5
     \              /
      3            3

Input: root = [8,null,5,null,3]
Output: [8,5,null,3]
Explanation: Every right child becomes a left child.
The chain now goes down to the left.

Example 3:

Input: root = [7]
Output: [7]
Explanation: A single node has no children to swap.

Constraints:

• 0 to 10,000 nodes.
• Each value is from 0 to 100.
• The tree is at most 400 levels deep.