（1）编程题系列 重新分布概率 Make a fair coin

You are given a function foo() that represents a biased coin. When foo() is called, it returns 0 with p probability, and 1 with 1-p probability. Write a new function that returns 0 and 1 with 50% probability each. Your function should use only foo(), no other library method.

关键是要找到一种情形，使得0和1的概率相同。有一种情形是，连续两次扔foo()，那出现01和出现10的概率是相同的，为p(1-p) = (1-p)p。

def foo():

def Fair_from_biased():
    first = foo()
    second = foo()
    if first == 0 and second == 1:
        return 0
    elif first == 1 and second == 0:
        return 1
    else:
        return Fair_from_biased()