菜单类

public static class Menu {
        Menu(String data) {
            String[] split = data.split(" ");
            this.id = Integer.valueOf(split[0]);
            this.name = split[1];
            this.pid = Integer.valueOf(split[2]);
        }
        Integer id;
        String name;
        Integer pid;
        List<Menu> children;
}

菜单数据

static String MENUS = "1 node1 0 ," +
            "2 node2 0 ," +
            "3 node3 0 ," +
            "4 node4 1 ," +
            "5 node5 1 ," +
            "6 node6 1 ," +
            "7 node7 2 ," +
            "8 node8 2 ," +
            "9 node9 3 ," +
            "10 node10 4 ," +
            "11 node11 4 ," +
            "12 node12 4 ," +
            "13 node13 5 ," +
            "14 node14 5 ," +
            "15 node15 6 ," +
            "16 node16 7 ," +
            "17 node17 7 ," +
            "18 node18 7 ," +
            "19 node19 8 ," +
            "20 node20 8 ," +
            "21 node21 9 ," +
            "22 node22 10 ," +
            "23 node23 10 ," +
            "24 node24 10 ," +
            "25 node25 11 ," +
            "26 node26 11 ," +
            "27 node27 12 ," +
            "28 node28 13 ," +
            "29 node29 13 ," +
            "30 node30 13 ," +
            "31 node31 14 ," +
            "32 node32 14 ," +
            "33 node33 15 ," +
            "34 node34 16 ," +
            "35 node35 16 ," +
            "36 node36 16 ," +
            "37 node37 17 ," +
            "38 node38 17 ," +
            "39 node39 18 ," +
            "40 node40 19 ," +
            "41 node41 19";

    public static List<Menu> getMenus(String s) {
        String[] split = s.split(" ,");
        List<Menu> menus = new ArrayList<>();
        for (String s1 : split) {
            menus.add(new Menu(s1));
        }
        return menus;
    }

两个循环出结果tree

    public static void main(String[] args) {
        List<Menu> menus = getMenus(MENUS);
        Map<Integer, List<Menu>> map = new HashMap();
        for (Menu menu : menus) {
            if (!map.containsKey(menu.getPid())) {
                map.put(menu.getPid(), new ArrayList<>());
            }
            map.get(menu.getPid()).add(menu);
        }
        for (Menu menu : menus) {
            menu.setChildren(map.get(menu.getId()));
        }
        List<Menu> res = map.get(0);
        System.out.println(res);
    }

结果图