无法从BST - C#中删除节点

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

class BST
{
    private TreeNode root;
    
    public BST()
    {
        this.root = null;
    }
    
    public void Insert(int val)
    {
        this.root = InsertNode(this.root, val);
    }
    
    private TreeNode InsertNode(TreeNode node, int val)
    {
        if (node == null)
        {
            return new TreeNode(val);
        }
        
        if (val < node.val)
        {
            node.left = InsertNode(node.left, val);
        }
        else if (val > node.val)
        {
            node.right = InsertNode(node.right, val);
        }
        
        return node;
    }
    
    public void Delete(int val)
    {
        this.root = DeleteNode(this.root, val);
    }
    
    private TreeNode DeleteNode(TreeNode node, int val)
    {
        if (node == null)
        {
            return null;
        }
        
        if (val < node.val)
        {
            node.left = DeleteNode(node.left, val);
        }
        else if (val > node.val)
        {
            node.right = DeleteNode(node.right, val);
        }
        else
        {
            if (node.left == null && node.right == null)
            {
                node = null;
            }
            else if (node.left == null)
            {
                node = node.right;
            }
            else if (node.right == null)
            {
                node = node.left;
            }
            else
            {
                TreeNode minNode = FindMinNode(node.right);
                node.val = minNode.val;
                node.right = DeleteNode(node.right, minNode.val);
            }
        }
        
        return node;
    }
    
    private TreeNode FindMinNode(TreeNode node)
    {
        while (node.left != null)
        {
            node = node.left;
        }
        
        return node;
    }
}