Company Organizational Structure

Generate a Mermaid flowchart in top-down (graph TD) layout to represent a company organizational structure.
The top-level node is "Headquarters". Under it, there are three departments: "Engineering", "Marketing", and "Human Resources".
"Engineering" has two sub-units: "Frontend Team" and "Backend Team".
"Marketing" has two sub-units: "Content Team" and "SEO Team".
"Human Resources" has one sub-unit: "Recruitment".
Show all parent-child relationships using arrows.