实现正序多级菜单展示，ztree树

	public List<MenuEntity> getMenuList() {
		List<MenuEntity> menuList = this.requestMapper.findAll();
		List<MenuEntity> menu = new ArrayList<>();
		HashMap<String, List<MenuEntity>> treeMap = new HashMap<>();
		//循环从数据库中获取的所有菜单
		for (MenuEntity menuEntity : menuList) {
			//筛选出所有子菜单
			if(menuEntity.getParentId() != null){
				//如果map中没有父菜单id
				if(!treeMap.containsKey(menuEntity.getParentId())){
					//将父菜单id当成map中的key，并且value为nul集合
					treeMap.put(menuEntity.getParentId(), new ArrayList<>());
				}
				//每次循环将按照parentid取值，再将当前parentid对应的实体放入map中value的位置
				treeMap.get(menuEntity.getParentId()).add(menuEntity);
			}
			
		}
		//循环从数据库中获取的所有菜单数据
		for (MenuEntity menuEntity : menuList) {
			//如果map中的parentid包含数据库中的菜单id
			if(treeMap.containsKey(menuEntity.getId())){
				List<MenuEntity> list = treeMap.get(menuEntity.getId());
				menuEntity.setMenuList(list);
			}
			//将组合好的菜单放入新的集合中一并返回
			if(menuEntity.getParentId() == null){
				menu.add(menuEntity);
			}
		}
		return menu;
	}