Hard 随机洗牌函数 @CareerCup

第i个元素和index在[i,length-1]之间的一个数随机交换

package Hard;

import CtCILibrary.AssortedMethods;

/**
 *
 * Write a method to shuffle a deck of cards. It must be a perfect shuffle - in other words, each 52! permutations of the deck has to be equally likely. Assume that you are given a random number generator which is perfect.

译文：

写一个随机洗牌函数。要求洗出的52!种组合都是等概率的。 也就是你洗出的一种组合的概率是1/(52!)。假设已经给你一个完美的随机数发生器。
 *
 */
public class S18_2 {
	/* Random number between lower and higher, inclusive */
    public static int rand(int lower, int higher) {
            return lower + (int)(Math.random() * (higher - lower + 1));
    }        

    // 递归shuffle，先shuffle前i-1个元素，然后把第i个元素和前面的一个随机元素交换
    public static int[] shuffleArrayRecursively(int[] cards, int i) {
            if (i == 0) {
            	return cards;
            }

            /* shuffle elements 0 through index - 1 */
            shuffleArrayRecursively(cards, i - 1);
            int k = rand(0, i);                

            /* Swap element k and index */
            int temp = cards[k];
            cards[k] = cards[i];
            cards[i] = temp;

            /* Return shuffled array */
            return cards;
     }

    public static void swap(int[] a, int n, int m){
    	int tmp = a[n];
    	a[n] = a[m];
    	a[m] = tmp;
    }

    // 遍历数组，对每一个在index为i上的元素，和index在[i,n-1]之间的一个随机index选择的数交换
    public static void shuffleArrayInteratively(int[] cards) {
    	for (int i = 0; i < cards.length; i++) {
        	int k = rand(i, cards.length-1);		// 产生i到n-1间的随机数
            swap(cards, i, k);
        }
    }

    public static void main(String[] args) {
            int[] cards = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
            System.out.println(AssortedMethods.arrayToString(cards));
            shuffleArrayInteratively(cards);
            System.out.println(AssortedMethods.arrayToString(cards));
    }
}