Javascript
/**
* Definition for a binary tree node.
*/
class TreeNode {
constructor(val, left, right) {
this.val = (val === undefined ? 0 : val)
this.left = (left === undefined ? null : left)
this.right = (right === undefined ? null : right)
}
}
/**
* @param {TreeNode} root
* @return {TreeNode}
*/
const invertTree = root => root !== null ? new TreeNode(root.val, invertTree(root.right), invertTree(root.left)) : null;