static void reverseTree(final TreeNode root) {
    if (root == null) {
        return;
    }
    final TreeNode temp = root.right;
    root.right = root.left;
    root.left = temp;
    reverseTree(root.left);
    reverseTree(root.right);
}

参考：https://stackoverflow.com/questions/9460255/reverse-a-binary-tree-left-to-right