Invert a Binary Tree

Problem Invert a binary tree — mirror it so every node's left and right subtrees are swapped — and return the root.

Input / Output

• Input: root of a binary tree (may be null).
• Output: the root of the mirrored tree.

Constraints

• 0 ≤ nodes ≤ 100.

Example

• [4,2,7,1,3,6,9] → [4,7,2,9,6,3,1].