c# - how to get parent id by using a child id in hierarchy?

Question

Item 01
  Item 01 01
  Item 01 02
     Item 01 02 01
     Item 01 02 02 <-----
     Item 01 02 03
  Item 01 03
Item 02
  Item 02 01
  Item 02 01

I'm using asp.net and C#. This is my main menu. I want to show some item of main menu in left sidebar menu. for example if a user selected [Item 01 02 02] i want to show all sub menu items of [Item 01] in left sidebar menu. I access to the id of [Item 01 02 02].

thanks for any idea.