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Obtaining a powerset of a set in Java

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青春惊慌失措
青春惊慌失措 2020-11-22 11:33

The powerset of {1, 2, 3} is:

{{}, {2}, {3}, {2, 3}, {1, 2}, {1, 3}, {1, 2, 3}, {1}}

Let\'s say I have a Set in Java:<

26条回答
  • 清歌不尽
    清歌不尽 (楼主)
    2020-11-22 12:07

    I was looking for a solution that wasn't as huge as the ones posted here. This targets Java 7, so it will require a handful of pastes for versions 5 and 6.

    Set> powerSetofNodes(Set orig) {
    Set> powerSet = new HashSet<>(),
        runSet = new HashSet<>(),
        thisSet = new HashSet<>();

    while (powerSet.size() < (Math.pow(2, orig.size())-1)) {
        if (powerSet.isEmpty()) {
            for (Object o : orig) {
                Set s = new TreeSet<>();
                s.add(o);
                runSet.add(s);
                powerSet.add(s);
            }
            continue;
        }
        for (Object o : orig) {
            for (Set s : runSet) {
                Set s2 = new TreeSet<>();
                s2.addAll(s);
                s2.add(o);
                powerSet.add(s2);
                thisSet.add(s2);
            }
        }
        runSet.clear();
        runSet.addAll(thisSet);
        thisSet.clear();
    }
    powerSet.add(new TreeSet());
    return powerSet;



    Here's some example code to test:
    


    Set hs = new HashSet<>();
hs.add(1);
hs.add(2);
hs.add(3);
hs.add(4);
for(Set s : powerSetofNodes(hs)) {
    System.out.println(Arrays.toString(s.toArray()));
}

    
    
             
    
                                                        
    
            
            
    
              
                
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