Exercise 5.1.3

$\star$ Suppose that you want to output $0$ with probability $1/2$ and 1 with probability $1/2$. At your disposal is a procedure BIASED-RANDOM that outputs either $0$ or $1$. It outputs $1$ with some probability $p$ and $0$ with probability $1 - p$, where $0 < p < 1$, but you don't now what $p$ is. Give an algorithm that uses BIASED-RANDOM as a subroutine, and returns an unbiased answer, returning $0$ with probability $1/2$ and $1$ with probability $1/2$. What is the expected running time of your algorithm as a function of $p$?


  1. Generate two random numbers $x$ and $y$
  2. If they are not the same, return $x$
  3. Otherwise, repeat from step 1

The change of getting [0, 1] is the same as getting [1, 0].

The likelyhood of that happening is $2pq$. Thus, the expected number of trials is $1/(2pq)$ making the running time $\O\Big(\frac{1}{p(1-p)}\Big)$.