Hard 随机选择subset @CareerCup

算法同上题

package Hard;

import CtCILibrary.AssortedMethods;

/**
 * Write a method to randomly generate a set of m integers from an array of size n. Each element must have equal probability of being chosen.

译文：

写一个函数，随机地从大小为n的数组中选取m个整数。要求每个元素被选中的概率相等。
 *
 */
public class S18_3 {

	/* Random number between lower and higher, inclusive */
    public static int rand(int lower, int higher) {
            return lower + (int)(Math.random() * (higher - lower + 1));
    }

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

    /* pick M elements from original array, using only elements 0 through i (inclusive).*/
    public static int[] pickMRecursively(int[] original, int m, int i) {
            if (i + 1 < m) { // Not enough elements
                return null;
            } else if (i + 1 == m) { // Base case -- copy first m elements into array
                int[] set = new int[m];
                for (int k = 0; k < m; k++) {
                    set[k] = original[k];
                }
                return set;
            } else {
                int[] set = pickMRecursively(original, m, i - 1);
                int k = rand(0, i);
                if (k < m) {
                    set[k] = original[i];
                }
                return set;
            }
    }

    /* pick M elements from original array.  Clone original array so that
     * we don’t destroy the input. */
    public static int[] pickMRandomly(int[] original, int m) {
        for (int i = 0; i < m; i++) {
        	int k = rand(i, original.length-1);		// 产生i到n-1间的随机数
            swap(original, i, k);
        }
        int[] subset = new int[m];
        for(int i=0; i<m; i++){
        	subset[i] = original[i];
        }
        return subset;
    }

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