等概率随机函数和任意概率随机函数

from peking2@mitbbs

题目：Given a function that will return 0 or 1 at the equal probability 0.5, called prob(), write a function biased_fun(double p), will return 1 with probability p.

我的一个面试题目和这个很相似

int equal_prob()
{
	return rand()%2;
}

int prob(double p, int target)
{
	if(p<0.5)
		return prob(1-p, !target);
	if(equal_prob() == target)
		return target;
	else
		return prob((p-0.5)*2, target);
}

另外这个题目还有一个变种：Given a biased_coin(), write a function that will return 0 or 1 with equal probability 0.5

int biased_coin();

int fair_coin()
{
	int first = biased_coin();
	int second = biased_coin();
	if(first==0 && second==1)
		return 1;
	if(first==1 && second==0)
		return 0;
	return fair_coin();
}