/**
 * Definition for a binary tree node.
 * function TreeNode(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}
 */
var invertTree = function(root) {
  if (root === null) { return null; }
  var node = root.left;
  root.left = root.right;
  root.right = node;
  if (root.left) { invertTree(root.left); }
  if (root.right) { invertTree(root.right); }
  return root;
};