And here some code! 👍
LevelTree tree = new MyArrayListTree();
tree.add("Level-1");
tree.add("Level-1", "Level-11");
tree.add("Level-1", "Level-12");
tree.add("Level-1", "Level-13");
tree.add("Level-12", "Level-121");
tree.add("Level-12", "Level-122");
tree.add("Level-122","Level-1221");
tree.add("Level-13","Level-132");
tree.add("Level-11","Level-111");
tree.add("Level-11", "Level-112");