Given the root node of a binary tree, flip the tree by swapping each node's left and right children, then return the root of the flipped tree.

The binary tree is represented by a collection of TreeNodes, where each node has optional left and right child nodes, which are also TreeNodes.

A TreeNode has the following interface:

interface TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
}

Input

• root: TreeNode: Root node of the tree. Examples display a level-order traversal of the tree

Input: root = [2,1,3]
Output: [2,3,1]
Explanation: The left and right children are swapped.

Input: root = [43,null,-55]
Output: [43,-55]
Explanation: The right child becomes the left child after inversion.

Input: root = [12,8,18,6,null,null,20]
Output: [12,18,8,20,null,null,6]
Explanation: The tree is inverted, with left and right children of each node being swapped.

Constraints

• 1 <= Number of nodes <= 100
• -100 <= TreeNode.val <= 100