Given the root of a binary tree, invert it so that all left and right children are swapped, and return the root.
With a tree:
4
/ \
2 7
/ \ / \
1 3 6 9
After inverting:
4
/ \
7 2
/ \ / \
9 6 3 1
Every node swaps its left and right subtrees. This happens at every level, recursively.
Constraints: nodes .