A randomized algorithm or probabilistic algorithm is an algorithm which employs a degree of randomness as part of its logic. In common practice, this means that the machine implementing the algorithm has access to a pseudorandom number generator. The algorithm typically uses the random bits as an auxiliary input to guide its behavior, in the hope of achieving good performance in the "average case". Formally, the algorithm's performance will be a random variable determined by the random bits, with (hopefully) good expected value; this expected value is called the expected runtime. The "worst case" is typically so unlikely to occur that it can be ignored.

Reference:
http://en.wikipedia.org/wiki/Randomized_algorithm
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