由网友(眉眼之间)分享简介：给定一组 {1,2,3,4,5 ... N} n个元素，我们需要找到一个长度为k的所有子集。 Given a set {1,2,3,4,5...n} of n elements, we need to find all subsets of length k . 例如，如果n = 4和k = 2，输出是 {1,2...

给定一组 {1,2,3,4,5 ... N} n个元素，我们需要找到一个长度为k的所有子集。

Given a set {1,2,3,4,5...n} of n elements, we need to find all subsets of length k .

例如，如果n = 4和k = 2，输出是 {1,2}，{1,3}， {1,4}，{2,3}，{2,4}，{3,4}

For example, if n = 4 and k = 2, the output would be {1, 2}, {1, 3}, {1, 4}, {2, 3}, {2, 4}, {3, 4}.

我甚至无法弄清楚如何下手。我们没有使用内置的库函数像next_permutation等

I am not even able to figure out how to start. We don't have to use the inbuilt library functions like next_permutation etc.

需要的算法和实现用C / C ++或Java。

Need the algorithm and implementation in either C/C++ or Java.

推荐答案

递归是你的朋友对这个任务。

Recursion is your friend for this task.

有关每个元素 - 猜测，如果它是在当前子集，并且与猜测递归调用和一个较小的超集可以从选择。这样做的同时为是和否猜测 - 将导致所有可能子集。 制约自己到一定长度可在停机条款很容易做到。

For each element - "guess" if it is in the current subset, and recursively invoke with the guess and a smaller superset you can select from. Doing so for both the "yes" and "no" guesses - will result in all possible subsets. Restraining yourself to a certain length can be easily done in a stop clause.

Java的code：

Java code:

private static void getSubsets(List<Integer> superSet, int k, int idx, Set<Integer> current,List<Set<Integer>> solution) {
    //successful stop clause
    if (current.size() == k) {
        solution.add(new HashSet<>(current));
        return;
    }
    //unseccessful stop clause
    if (idx == superSet.size()) return;
    Integer x = superSet.get(idx);
    current.add(x);
    //"guess" x is in the subset
    getSubsets(superSet, k, idx+1, current, solution);
    current.remove(x);
    //"guess" x is not in the subset
    getSubsets(superSet, k, idx+1, current, solution);
}

public static List<Set<Integer>> getSubsets(List<Integer> superSet, int k) {
    List<Set<Integer>> res = new ArrayList<>();
    getSubsets(superSet, k, 0, new HashSet<Integer>(), res);
    return res;
}

与调用：

List<Integer> superSet = new ArrayList<>();
superSet.add(1);
superSet.add(2);
superSet.add(3);
superSet.add(4);
System.out.println(getSubsets(superSet,2));

将产生：

[[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]]