GATE | GATE-CS-2014-(Set-3) | Question 65

Consider a hash table with 100 slots. Collisions are resolved using chaining. Assuming simple uniform hashing, what is the probability that the first 3 slots are unfilled after the first 3 insertions?
(A) (97 × 97 × 97)/1003
(B) (99 × 98 × 97)/1003
(C) (97 × 96 × 95)/1003
(D) (97 × 96 × 95)/(3! × 1003)


Answer: (A)

Explanation: Simple Uniform hashing function is a hypothetical hashing function that evenly distributes items into the slots of a hash table. Moreover, each item to be hashed has an equal probability of being placed into a slot, regardless of the other elements already placed. (Source: https://en.wikipedia.org/wiki/SUHA_%28computer_science%29).

Probability that the first 3 slots are unfilled after the first 3 insertions = 
                (probability that first item doesn't go in any of the first 3 slots)*
                (probability that second item doesn't go in any of the first 3 slots)*
                (probability that third item doesn't go in any of the first 3 slots)

                 = (97/100) * (97/100) * (97/100) 


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