/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} node
 * @return {TreeNode}
 */
var invertTree = function(node) {
  if (node) [node.left, node.right] = [invertTree(node.right), invertTree(node.left)];
  return node;
};