1. public TreeNode invertTree(TreeNode root) {
    2.     preOrder(root);
    3.     return root;
    4. }
    6. private void preOrder(TreeNode node) {
    7.     if (node != null) {
    8.         TreeNode oldLeft = node.left;
    9.         node.left = node.right;
    10.         node.right = oldLeft;
    11.         preOrder(node.left);
    12.         preOrder(node.right);
    13.     }
    14. }