Saturday, December 29, 2018

LightOJ 1248 - Dice (III)

Topic : Probability/Expected value/Geometric Distribution

Short Discription :
Given a dice with n sides, you have to find the expected number of times you have to throw that dice to see all its faces at least once. Assume that the dice is fair, that means when you throw the dice, the probability of occurring any face is equal.

Idea:
First of all, the probability of coming the first unexposed face of the dice is    , as still no face has come up before it.
Than the probability of coming the second unexposed face of the dice is    , as one face has come up already.
Similarly, the probability of coming the ith unexposed face of the dice is   
.
This is clearly a geometric distribution.
We know expected value in geometry distribution is ,  

so, our expected result    

Solution:


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