title: Building a tree using a list of objects
body: <p>I have a list of objects with property id and parent_id.<br>
  I want to build a tree to link up those children and parents.<br>
  1 parent may have several children and there is an object which will be the ancestor of all objects.</p>
  
  <p>What's the fastest algorithm to implement that?<br>
  I use C# as programming language, but other languages are also okay.</p>