获取列表元素的组合列表 [英] Get a list of combinations of lists' elements

问题描述

假设我有3个列表：['q'，'w']，['a'，'s']，['z'，'x']。如何从这些列表中获取可能的组合列表？所以我得到一个列表[['q'，'a'，'z']，['q'，'s'，'z']]等等。我为两个方法制作了一个方法，但是不能为N个列表找到一个方法：

Suppose I have 3 lists: ['q','w'], ['a','s'], ['z','x']. How to get a list of possible combinations out of these lists? So I get a list [['q','a','z'],['q','s','z']] and such. I made a method for two, but can't figure one for N lists:

static <E> ArrayList combine(ArrayList<E> one,ArrayList<E> two)
{
    ArrayList<ArrayList<E>> combs=new ArrayList<ArrayList<E>>();
    for(E e:one)
    {
        for(E e2:two)
        {
            ArrayList ps=new ArrayList();
            ps.add(e);
            ps.add(e2);
            combs.add(ps);
        }
    }
    return combs;
}

我发现这是由Guava的Sets.cartesianProduct完成的。​​

I found out that this is done by Guava's Sets.cartesianProduct.

推荐答案

你需要N个嵌套循环，这就是它的难点。

You need N nested loops, which is what makes it hard.

你可以使用递归来实现这一点。

You can achieve this using recursion though.

static <E> ArrayList combine(ArrayList<E> soFar, ArrayList<E>... lists)
{
    // Rather than constantly making and remaking this list could just use one
    // and pass it around and add stuff to it. This works though.
    ArrayList<ArrayList<E>> combs=new ArrayList<ArrayList<E>>();

    // Loop through the first list looking for elements
    for(E e:lists[0])
    {
       // Create a new List to build this combination
       ArrayList<E> temp = new ArrayList<>(soFar);
       // Add this element to the combination
       temp.add(e);
       // If there are more lists recurse down
       if (lists.length > 1) {
           // Use recursion to add all combinations of the remaining lists
           combs.addAll(combine(temp, lists.subList(1)));
       } else {
           // There are no more lists so we are done, add temp to the combos
           combs.add(temp);
       }
    }
    return combs;
}


// Call this method to start things going, the other one should probably be private
static <E> ArrayList combine(ArrayList<E>... lists)
    return combine(new ArrayList<E>(), lists);
}

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