[19.10] Implement rand7() using rand5()

1. Example
rand7()
(0-6)+1 =(1-7)
Generate 0~6 from rand5()


rand2()
-> 1/2 probability to be 0
-> 1/2 probability to be 1

(0-1)+1 = (1-2)

rand7()
(0-6)+1 =(1-7)

(0,6) <= (1-21)%7=1,2,3,4,5,6,0,1,...21%7=0 = > 0~6

rand2() to generate 3 the probability is 3 since it should be as random as rand3()
rand3()

2. Implementation

public static int rand7()
{
     while (true)
     {
           // 5* (0~4) + (0~4) = (0~20)+(0~4) = (0~24)
           int num = 5*( rand5() -1 ) + ( rand5() -1 );
           if (  num < 21 )
           {
                // ( 0~20 ) %7
                //   0,1,2,3,4,5,6,0,1,2,3,4,5.....18%7=4,19%7=5,20%7=6
                return (num%7+1);
           }
     }
}

3. Similar ones