public class  {
    class TreeNode{
        int value;
        TreeNode left;
        TreeNode right;
    }
    public void mirror(TreeNode root) {
        if(root == null){
            return;
        }
        if(root.left == null && root.right == null){
            return;
        }
        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;
        if(root.left != null){
            mirror(root.left);
        }
        if(root.right != null){
            mirror(root.right);
        }
    }
}