java - The union of all combinations of n sets?

Question

In this question The intersection of all combinations of n sets a method based on Map#reduce is presented to obtain all intersections of n sets. My question is, is there such a way to find the union of n sets?

This is what I have done, but it is very slow for a large number of sets:

public static void main(String[] args) {
    List<List<Set<Integer>>> unionSet = new ArrayList<>();
    List<List<Integer>> sets = ...
    double avail = 0;
    for (int i = 1; i <= sets.size(); i++) {
        List<Set<Integer>> us = new ArrayList<>();
        union(sets, us, new HashSet<>(), i, 0);
        unionSet.add(us);
    }
}

public static void union(List<List<Integer>> sets, List<Set<Integer>> unionSet,
                         Set<Integer> set, int size, int index) {
    for (int i = index; i < sets.size(); i++) {
        Set temp = new HashSet(set);
        temp.addAll(set.get(i));
        if (size != 1)
            union(sets, unionSet, temp, size - 1, i + 1);
        else
            unionSet.add(temp);
    }
}

question from:https://stackoverflow.com/questions/65914891/the-union-of-all-combinations-of-n-sets

Answer 1

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