Convert hierarchical list to a flat list in java

Question

I have a hierarchical list like below and I want to convert it to a flat list.

I have wrote a method called convertToFlatList

Answer 1

Do this by using java-8 flatMap and recursion

Change convertToFlatList method to take only one argument (i,e List)

If getChildren() is not null, then do the recursion or else return the current object

private static List convertToFlatList(List memberList)
  {
       return memberList.stream().flatMap(i->{
          if(Objects.nonNull(i.getChildren())) {
             return Stream.concat(Stream.of(i), convertToFlatList(i.getChildren()).stream());
          }
          return Stream.of(i);

      }).collect(Collectors.toList());

  }   

Output :

[1, 2, 3, 4, 5, 6, 7]