List inputList = new ArrayList<>();
inputList.add("a");
List bList = new ArrayList<>();
bList.add("b1");
bList.add("b2");
inputList.add(bList);
inputList.add("c");
List bList2 = new ArrayList<>();
bList2.add("b3");
bList2.add("b4");
inputList.add(bList2);
List flattenedList = flatten(inputList);
System.out.println(flattenedList);