在Java中寻找笛卡尔乘积

Question

我想找一组元素的笛卡尔乘积。下面是一个例子

示例1：

sets : (ab) (bc) (ca)

笛卡尔乘积是：

abc aba acc aca bbc bba bcc bca

示例2：

sets : (zyx) b c

笛卡尔乘积是：

zbc ybc xbc

因此，我正在考虑一种在Java中执行的算法，它可以找到在编译时定义的特定数量组的笛卡尔乘积。

Answer 1

您可以使用Google's Guava libraries中的Sets.cartesianProduct()方法生成笛卡尔积：

com.google.common.collect.Sets.cartesianProduct(Set[] yourSets)

如果一切都那么简单就好了！

Answer 2

定义您自己的迭代器/迭代器：

import java.util.*;

class CartesianIterator <T> implements Iterator <List <T>> {

    private final List <List <T>> lilio;    
    private int current = 0;
    private final long last;

    public CartesianIterator (final List <List <T>> llo) {
        lilio = llo;
        long product = 1L;
        for (List <T> lio: lilio)
            product *= lio.size ();
        last = product;
    } 

    public boolean hasNext () {
        return current != last;
    }

    public List <T> next () {
        ++current;
        return get (current - 1, lilio);
    }

    public void remove () {
        ++current;
    }

    private List<T> get (final int n, final List <List <T>> lili) {
        switch (lili.size ())
        {
            case 0: return new ArrayList <T> (); // no break past return;
            default: {
                List <T> inner = lili.get (0);
                List <T> lo = new ArrayList <T> ();
                lo.add (inner.get (n % inner.size ()));
                lo.addAll (get (n / inner.size (), lili.subList (1, lili.size ())));
                return lo;
            }
        }
    }
}

class CartesianIterable <T> implements Iterable <List <T>> {

    private List <List <T>> lilio;  

    public CartesianIterable (List <List <T>> llo) {
        lilio = llo;
    }

    public Iterator <List <T>> iterator () {
        return new CartesianIterator <T> (lilio);
    }
}

并使用您的数据进行测试：

class CartesianIteratorTest {

    public static void main (String[] args) {
        List <Character> la = Arrays.asList (new Character [] {'a', 'b'});
        List <Character> lb = Arrays.asList (new Character [] {'b', 'c'});      
        List <Character> lc = Arrays.asList (new Character [] {'c', 'a'});
        List <List <Character>> llc = new ArrayList <List <Character>> ();
        llc.add (la);
        llc.add (lb);
        llc.add (lc);

        CartesianIterable <Character> ci = new CartesianIterable <Character> (llc);
        for (List<Character> lo: ci)
            show (lo);

        la = Arrays.asList (new Character [] {'x', 'y', 'z'});
        lb = Arrays.asList (new Character [] {'b'});    
        lc = Arrays.asList (new Character [] {'c'});
        llc = new ArrayList <List <Character>> ();
        llc.add (la);
        llc.add (lb);
        llc.add (lc);

        ci = new CartesianIterable <Character> (llc);
        for (List<Character> lo: ci)
            show (lo);    
    }

    public static void show (List <Character> lo) {
        System.out.print ("(");
        for (Object o: lo)
            System.out.print (o);
        System.out.println (")");
    }
}

结果：

(abc)
(bbc)
(acc)
(bcc)
(aba)
(bba)
(aca)
(bca)
(xbc)
(ybc)
(zbc)