1、洗牌

private static void swap(int[] arr, int i, int j){
       int tmp = arr[i];
       arr[i] = arr[j];
       arr[j] = tmp;
}
public static void shuffle(int[] arr){
    Random rnd = new Random();
    for(int i=arr.length; i>1; i--) {
    swap(arr, i-1, rnd.nextInt(i));
  }
}

//主方法
main(){
   int[] arr = new int[13];
   for(int i=0; i<arr.length; i++){ 
               arr[i] = i;
  }
    shuffle(arr);
    System.out.println(Arrays.toString(arr));
}

 

2、帶權重的隨即選擇
業務場景：1元、5元和10元，權重分別為70、20、10

class Pair {
     Object item;
     int weight;
     public Pair(Object item, int weight){
		this.item = item;
		this.weight = weight;
    }
	public Object getItem() {
		return item;
    }
	public int getWeight() {
	return weight;
   }
}


public class WeightRandom {
		private Pair[] options;
		private double[] cumulativeProbabilities;
		private Random rnd;
public WeightRandom(Pair[] options){
		this.options = options;
		this.rnd = new Random();
		prepare();
}
private void prepare(){
		int weights = 0;
		for(Pair pair : options){
		weights += pair.getWeight();
		}
		cumulativeProbabilities = new double[options.length];
		int sum = 0;
		for(int i = 0; i<options.length; i++) {
		sum += options[i].getWeight();
		cumulativeProbabilities[i] = sum / (double)weights;
		}
		}
		public Object nextItem(){
		double randomValue = rnd.nextDouble();
		int index = Arrays.binarySearch(cumulativeProbabilities, randomValue);
		if(index < 0) {
		index = -index-1;
		}
		return options[index].getItem();
		}
}

//主方法
 main(){
		Pair[] options = new Pair[]{
		new Pair("1元",7), new Pair("2元", 2), new Pair("10元", 1)
		};
		WeightRandom rnd = new WeightRandom(options);
		for(int i=0; i<10; i++){
		System.out.print(rnd.nextItem()+" ");
		}
}
 

3、搶紅包演算法

public class RandomRedPacket {
private int leftMoney;
private int leftNum;
private Random rnd;
public RandomRedPacket(int total, int num){
this.leftMoney = total;
this.leftNum = num;
this.rnd = new Random();
}
public synchronized int nextMoney(){
if(this.leftNum<=0){
throw new IllegalStateException("???");
}
if(this.leftNum==1){
return this.leftMoney;
}
double max = this.leftMoney/this.leftNum*2d;
int money = (int)(rnd.nextDouble()*max);
money = Math.max(1, money);
this.leftMoney -= money;
this.leftNum --;
return money;
}
}

//主方法
main(){
	RandomRedPacket redPacket = new RandomRedPacket(1000, 10);
	for(int i=0; i<10; i++){
	  System.out.print(redPacket.nextMoney()+" ");
  }
}
 

整理自《JAVA程式設計的邏輯》