使用函数rand5()来实现函数rand7()

int Rand5()
{
	return rand()%5+1;
}
int Rand7()
{
	int ret;
	while(1)
	{
		ret = (Rand5()-1)*5 + Rand5(); ////产生[1,25]的整数区间
		if(ret >= 22)  // //将[1,25]整数区间控制于[1,21]
			continue;
		else
			return ret%7+1; //  //将[1,21]映射到[1,7]
	}
}

https://www.cnblogs.com/zkfopen/p/11187827.html

https://blog.youkuaiyun.com/qq_42719751/article/details/104458219

https://www.i3geek.com/archives/832

[1,21]的原因是为了等概率生成1-7的数