Invert Binary Tree

public class Solution {
    public TreeNode invertTree(TreeNode root) {
        helper(root);
        return root;
    }
    public void helper(TreeNode root)
    {
        if(root == null)    return;
        TreeNode left = root.left;
        TreeNode right = root.right;
        root.left = right;
        root.right = left;
        helper(left);
        helper(right);
    }
}