Fisher-Yates (java) 与 Collections.shuffle

Question

我还是个初学者，所以请善待。我很难理解为什么 Fisher-Yates 算法会永久修改数组元素而 Collections 的 shuffle 方法不会。我怀疑一个原因是我可能实际上并没有正确理解 Fisher-Yates 算法的工作原理，但我将不胜感激。代码如下：

导入 java.util.*;

公共类 ArrayShuffle {

private static char[] letters = {'a','b','c','d','e','f','g'};

public static void main(String[] args) {
    System.out.println("Collections' shuffle method:");
    shuffleCollections(letters);
    System.out.print(letters);
    System.out.println();
    System.out.println("Fisher-Yates method:");
    shuffleFisherYates(letters);
    System.out.print(letters);

}

public static void shuffleCollections(char[] abc) {
    List letterList = new ArrayList();
    for(int alph=0; alph<abc.length; alph++)
        //convert char to ArrayList object
        letterList.add(abc[alph]);
    //shuffle
    Collections.shuffle(letterList);
    System.out.println(letterList);


}

public static void shuffleFisherYates(char[] abc) {
    int size = abc.length;
    Random random = new Random();
    for(int alph=0; alph<abc.length; alph++) {
        int randomIndex = alph + random.nextInt(size - alph);
        char randomLetter = abc[randomIndex];
        abc[randomIndex] = abc[alph];
        abc[alph] = randomLetter;
    }
    for(int shuffled = 0; shuffled<abc.length; shuffled++)
        System.out.print(abc[shuffled]+" ");
    System.out.println();

}

}

Answer 1

您的shuffleFisherYates 直接修改输入数组（abc[someIndex] = ...; 就是这样做的）。

您的shuffleCollections 基于输入数组构建ArrayList，然后修改ArrayList（通过调用Collections.shuffle(letterList)）。它不会修改输入数组。