java求解 集合的子集

方式1:我们知道子集个数 2的n次方

比如a,b,c的子集

     * 000  0  {}
     *001  1   a
     *010  2   b
     *011  3   a,b (b,a)
     *100  4   c
     * 101  5   a,c (c,a)
     * 110  6   b,c (c,b)
     * 111  7   a,b,c

利用二进制的对应关系

@Testpublic 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);}//集合接受各种类型数据public  <T> Set<ArrayList<T>> getSubsets(List<T> subList) {//考虑去重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;}

方式2:归纳法

      @Testpublic void testName() throws Exception {Set<List<Integer>> subsets2 = getSubsets2(Arrays.asList(1,2,3));System.out.println(subsets2);}//方式2  归纳法//从{}和最后一个元素开始,每次迭代加一个元素组成一个新的集合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;}        //这样写可以不影响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;}