运动场上有很多组照明设备，下面A集合中，描述出了不同照明设备的启动（begintime）停止（endtime）时间，照明设备的id已经处理掉了无需考虑，仔细看这些时间段可以发现，这些时间有重合（表示这段时间多组照明同时亮）、有空挡（表示这段时间一组照明都没有开）。请通过java代码实现，获得此足球场上有照明设备开启的时间段集合（求A集合在时间维度上的并集）。收起 A集合： [{begintime:2020-01-01 00:04:00,endtime:2020-01-01 00:06:00}, {begintime:2020-01-01 00:01:05,endtime:2020-01-01 00:03:15}, {begintime:2020-01-01 00:09:00,endtime:2020-01-01 00:10:00}, {begintime:2020-01-01 00:07:35,endtime:2020-01-01 00:08:40}, {begintime:2020-01-01 00:12:00,endtime:2020-01-01 00:17:00}, {begintime:2020-01-01 00:12:05,endtime:2020-01-01 00:13:10}, {begintime:2020-01-01 00:19:00,endtime:2020-01-01 00:21:00}, {begintime:2020-01-01 00:15:20,endtime:2020-01-01 00:20:45}]  期望得到的输出结果： [{begintime:2020-01-01 00:01:05,endtime:2020-01-01 00:03:15}, {begintime:2020-01-01 00:04:00,endtime:2020-01-01 00:06:00}, {begintime:2020-01-01 00:07:35,endtime:2020-01-01 00:08:40}, {begintime:2020-01-01 00:09:00,endtime:2020-01-01 00:10:00}, {begintime:2020-01-01 00:12:00,endtime:2020-01-01 00:21:00}]

可以使用以下代码实现：

import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<Map<String, String>> A = new ArrayList<Map<String, String>>();
        A.add(new HashMap<String, String>() {{
            put("begintime", "2020-01-01 00:04:00");
            put("endtime", "2020-01-01 00:06:00");
        }});
        A.add(new HashMap<String, String>() {{
            put("begintime", "2020-01-01 00:01:05");
            put("endtime", "2020-01-01 00:03:15");
        }});
        A.add(new HashMap<String, String>() {{
            put("begintime", "2020-01-01 00:09:00");
            put("endtime", "2020-01-01 00:10:00");
        }});
        A.add(new HashMap<String, String>() {{
            put("begintime", "2020-01-01 00:07:35");
            put("endtime", "2020-01-01 00:08:40");
        }});
        A.add(new HashMap<String, String>() {{
            put("begintime", "2020-01-01 00:12:00");
            put("endtime", "2020-01-01 00:17:00");
        }});
        A.add(new HashMap<String, String>() {{
            put("begintime", "2020-01-01 00:12:05");
            put("endtime", "2020-01-01 00:13:10");
        }});
        A.add(new HashMap<String, String>() {{
            put("begintime", "2020-01-01 00:19:00");
            put("endtime", "2020-01-01 00:21:00");
        }});
        A.add(new HashMap<String, String>() {{
            put("begintime", "2020-01-01 00:15:20");
            put("endtime", "2020-01-01 00:20:45");
        }});

        List<Map<String, String>> result = new ArrayList<Map<String, String>>();
        Collections.sort(A, new Comparator<Map<String, String>>() {
            public int compare(Map<String, String> a, Map<String, String> b) {
                return a.get("begintime").compareTo(b.get("begintime"));
            }
        });

        String beginTime = A.get(0).get("begintime");
        String endTime = A.get(0).get("endtime");
        for (int i = 1; i < A.size(); i++) {
            String currBeginTime = A.get(i).get("begintime");
            String currEndTime = A.get(i).get("endtime");
            if (currBeginTime.compareTo(endTime) <= 0) {
                endTime = currEndTime.compareTo(endTime) > 0 ? currEndTime : endTime;
            } else {
                result.add(new HashMap<String, String>() {{
                    put("begintime", beginTime);
                    put("endtime", endTime);
                }});
                beginTime = currBeginTime;
                endTime = currEndTime;
            }
        }
        result.add(new HashMap<String, String>() {{
            put("begintime", beginTime);
            put("endtime", endTime);
        }});

        System.out.println(result);
    }
}

该代码的基本思路是：首先对 A 集合按照 `begintime` 进行排序，然后遍历 A 集合，如果当前时间段与前一个时间段有重合，则更新当前时间段的结束时间，否则将前一个时间段添加到结果集中，然后将开始时间和结束时间更新为当前时间段的开始时间和结束时间。最后将最后一个时间段添加到结果集中即可。