題目

實現一個抽獎功能。共有N中獎品，每種獎品都有一個對應的權重（整數），該獎品被抽到的概率與其權重成正比。假設獎品i的權重為weights[i]，0 <= i < N，wieghts是長度為N的int陣列，實現以下函式，返回抽中的獎品的編號。
舉例：假設weights的內容為{10,20,20}，那麼應該有20%的概率返回0號獎品，40%的概率返回1號獎品，40%的概率返回2號獎品。
注：你可以使用的隨機數相關的系統函式為int rand(int n)，此函式返回值為[0,n)之間均勻分佈的一個隨機整數。
int randomByWeight(int[] weights)
{

}

程式碼

using System;



public class Test
{
    public static void Main()
    {
        int[] weights = new int[]{5, 5, 3, 1};
        int number = RandomByWeight(weights);
        Console.WriteLine("return prize is {0}", number);
    }

    public static int RandomByWeight(int[] weights)
    {
        for
 
(int i = 1; i < weights.Length; i++)
        {
            weights[i] += weights[i-1];
            Console.WriteLine("weights[{0}] = {1}", i, weights[i]);
        }
        int sum = weights[weights.Length-1];
        int number_rand = rand(sum);
        Console.WriteLine("number_rand = {0}", number_rand);

        if
 
(0 <= number_rand && number_rand < weights[0])
            {Console.WriteLine("{0} <= {1} < weights[0]={2}", 0, number_rand, weights[0]);return 0;}
        for(int i = 1; i < weights.Length; i++)
        {
            if(weights[i-1] <= number_rand && number_rand < weights[i])
            {
                Console.WriteLine("weights[{0}]={1} <= {2} < weights[{3}]={4}", i-1, weights[i-1], number_rand, i, weights[i]);
                return i;
            }
        }
        return -1;
    }

    static int rand(int n)
    {
        Random rd = new Random();
        return rd.Next(0, n);
    }
}

輸出

weights[1] = 10
weights[2] = 13
weights[3] = 14
number_rand = 12
weights[1]=10 <= 12 < weights[2]=13
return reward is 2

補充

• 將 0 <= number_rand && number_rand < weights[0] 這種情況單獨拿出來，避免在for迴圈中判斷影響效率。

改進後的程式碼

using System;

public class Test
{
    public static void Main()
    {
        int[] table = new int[]{10, 20, 30, 40, 50};
        Console.WriteLine("result = " + RandomByWeight(table));
    }

    public static int RandomByWeight(int[] weights)
    {
        int sum = 0;
        for(int i = 0; i < weights.Length; i++)
        {
            sum += weights[i];
        }
        int number_rand = rand(sum);
        Console.WriteLine("number_rand = " + number_rand);

        int sum_temp = 0;

        for(int i = 0; i < weights.Length; i++)
        {
            sum_temp += weights[i];
            if(number_rand <= sum_temp){return i;}
        }

        return -1;
    }

    static int rand(int n)
    {
        Random rd = new Random();
        return rd.Next(0, n);
    }
}

輸出

number_rand = 46
result = 2