列表的可能组合

问题描述

我有一个对象的arraylist，我想创建所有可能的组合（根据一组简单的规则）。存储在列表中的每个对象都包含一个squadNumber和一个字符串。以下是我存储的典型列表示例：

I have an arraylist of objects that I want to create all possible combinations (according to a simple set of rules). Each object that is stored in the list holds a squadNumber and a string. Here is an example of a typical list I am storing:

0: 1, A
1: 1, B
2: 2, A
3: 2, B
4: 3, C
5: 3, D
6: 4, C
7: 4, D

我想获得每个squadNumber只能出现一次的所有组合，例如：（1，A），（2，A），（3，C），（4，C）那么下一个组合将是（1，A），（2，A），（3，C），（ 4，d）。
我如何在java中解决这个问题？通常我会使用嵌套循环，但它全部存储在一个列表中这一事实使我感到困惑。

I want to get all the combinations where each squadNumber can only be present once, for example: (1,A),(2,A),(3,C),(4,C) then the next combination would be (1,A),(2,A),(3,C),(4,D).How would I go about this in java? Usually I would use a nested loop, but the fact that it's all being stored in one list complicates things for me.

谢谢，
paintstripper

Thanks,paintstripper

推荐答案

已编辑

算法如下：

1. 按数字拆分所有小队。所以我们有小队的名单为1，
   小队2的另一个名单等等。


2. 运行dfs。在第n步，我们添加第n个小队。

代码

// Split squads by numbers, so we can iterate through each number independently.
private Map<Integer, List<Squad>> splitSquadsByNumbers(List<Squad> squads) {
    Map<Integer, List<Squad>> res = new HashMap<Integer, List<Squad>>();
    for (Squad squad : squads) {
        if (res.get(squad.getNumber()) == null) {
            res.put(squad.getNumber(), new ArrayList<Squad>());
        }
        res.get(squad.getNumber()).add(squad);
    }
    return res;
}

List<Integer> squadNumbers;
Map<Integer, List<Squad>> squadsByNumbers;
Stack<Squad> stack;

// Iterating through each squad with number squadNumbers[position] and try to add to stack, at the end pop it from stack.

private void dfs(int position) {
    if (position == squadNumber.size()) {
        System.out.println(stack.toString());
    } else {
        for (Squad squad : squadsByNumbers.get(squadNumber.get(position))) {
            stack.push(squad);
            dfs(position + 1);
            stack.pop();
        }
    }
}

private void main(List<Squad> squads) {
    squadsByNumbers = splitSquadsByNumbers(squads);
    squadNumber = new ArrayList(squadsByNumber.keySet());
    Collections.sort(squadNumbers);
    stack = new Stack<Squad>();
    dfs(0);
}

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