Randomized Dictionary Structures:Structural Properties of Skip Lists

Structural Properties of Skip Lists

Number of Levels in Skip List

Recall that r = 1 + maxi{Zi}. Notice that Zi k iff the coin tossed for ki gets a run of at least k successes right from the beginning and this happens with probability pk . Since r k iff at least one of Zi k 1 , we easily get the following fact from Boole’s inequality


Choosing k = 4 log1/p n + 1, we immediately obtain a high confidence bound for the number of levels. In fact,


We obtain an estimation for the expected value of r, using the formula stated in theorem ( 13.3) as follows:


THEOREM 13.11 The expected number of levels in a skip list of n elements is O(log n). In fact, the number is O(log n) with high probability.

Space Complexity

Recall that the space complexity, | SL | is given by


Since Zi is a geometric random variable with parameter p, Z is a negative binomial random variable by theorem 13.6.


This implies that 4n is in fact a very high confidence bound for Z. Since | SL |= Z+n+2r+2, we easily conclude that

THEOREM 13.12 The space complexity of a skip list for a set of size n is O(n) with very high probability.

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