C#插入函数到BST中

public class TreeNode
{
    public int val;
    public TreeNode left;
    public TreeNode right;

    public TreeNode(int val)
    {
        this.val = val;
        left = null;
        right = null;
    }
}

public class BST
{
    public TreeNode root;

    public BST()
    {
        root = null;
    }

    public void Insert(int val)
    {
        root = InsertNode(root, val);
    }

    private TreeNode InsertNode(TreeNode root, int val)
    {
        if (root == null)
        {
            root = new TreeNode(val);
            return root;
        }

        if (val < root.val)
        {
            root.left = InsertNode(root.left, val);
        }
        else if (val > root.val)
        {
            root.right = InsertNode(root.right, val);
        }

        return root;
    }
}

BST bst = new BST();
bst.Insert(5);
bst.Insert(3);
bst.Insert(7);