@Test public void test1() throws Exception {      Set<ArrayList<Integer>> subsets = getSubsets( Arrays.asList(1,2,6));   Set<ArrayList<String>> subsets2 = getSubsets( Arrays.asList("a","b","c"));   Set<ArrayList<Character>> subsets3 = getSubsets( Arrays.asList('b','c','d'));   System.out.println(subsets);   System.out.println(subsets2);   System.out.println(subsets3); }  //集合接受各種類型數據 public <T> Set<ArrayList<T>> getSubsets(List<T> subList) {   //考慮去重   Set<ArrayList<T>> allsubsets = new LinkedHashSet<>();   int max = 1 << subList.size();   for (int loop = 0; loop < max; loop++) {     int index = 0;     int temp = loop;     ArrayList <T> currentCharList = new ArrayList<T>();     //控制索引     while (temp > 0) {       if ((temp & 1) > 0) {         currentCharList.add(subList.get(index));       }       temp >>= 1;       index++;     }     allsubsets.add(currentCharList);   }   return allsubsets; } 

   @Test public void testName() throws Exception {   Set<List<Integer>> subsets2 = getSubsets2(Arrays.asList(1,2,3));   System.out.println(subsets2); }  //方式2 歸納法 //從{}和最后一個元素開始,每次迭代加一個元素組成一個新的集合 public  Set<List<Integer>> getSubsets2(List<Integer> list) {    if (list.isEmpty()) {      Set<List<Integer>> ans=new LinkedHashSet<>();      ans.add(Collections.emptyList());      return ans;    }        Integer first=list.get(0);    List<Integer> rest=list.subList(1, list.size());    Set<List<Integer>> list1 = getSubsets2(rest);    Set<List<Integer>> list2 = insertAll(first, list1);//    System.out.println(list1);    System.out.println(list2);    System.out.println("================");    return concat(list1, list2); }      public  Set<List<Integer>> insertAll(Integer first,Set<List<Integer>> lists){   //   Set<List<Integer>> result=new LinkedHashSet<>();   for (List<Integer> list : lists) {     List<Integer> copy=new ArrayList<>();     copy.add(first);     copy.addAll(list);     result.add(copy);   }   return result; }      //這樣寫可以不影響lists1,lists2的值   private Set<List<Integer>> concat(Set<List<Integer>> lists1,Set<List<Integer>> lists2) {   Set<List<Integer>> temp=new LinkedHashSet<>(lists1);   temp.addAll(lists2);   return temp; }