RAND2()去实现RAND6()
int Rand6()
{
while(1)
{
int r=3*rand2()+rand2();
if(r<6)return r;
}
}
###cc150 17.11 用rand5()产生rand7()
```C++
int rand7()
{
while(1)
{
int num=5*rand5()+rand5();
if(num<21)return num%7;
}
}