java - Algorithm/function that returns random number in array in O(1) and creates single permutation -

this question has answer here:

• random shuffling of array 18 answers

intro: interview question had couldn't solve. code solution (in language), algorithm/pseudocode great.

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the problem: problem designing algorithm solves following problem:
given function int getrand(int x) gets int x , returns int in range 0 x randomly (exclusively -> [0, k) ). each call getrand() performed in o(1) time. given array int[] arr of size k containing integers.

write function getrandunique() when called return random member arr such after k requests have full permutation of members of arr (this means getrandunique() return different member of arr each call). each call getrandunique() has performed in o(1) time.
allowed use/store global variables etc...

e.g.: assume arr = [2, 3, 5 , 1]. call getrandunique() return either 2, 3, 5, 1 in 1/4 probability. consequent call getrandunique() return on of remaining 3 members in 1/3 probability , on...

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attempt: solved myself (after trial , error) , posted solution "q&a style". love other possible ideas/solutions. accept solution works correct answer (i don't want accept own)!

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question: how can achieved above restrictions?

edit: aware problem corresponds fisher–yates shuffle, though specifications little different/more strict here.

my solution follows:

• define index = 0.

• call , assign index = getrand(k) (remember k size of arr).

• swap arr[index] arr[k].

• now call , assign index = getrand(k-1). can won't index k again won't have remove it.
• swap arr[index] arr[k-1]
• continue doing until call last getrand(1).

now have array arr random permutation of requested (don't need additional array).