README
No arithmetic progressions
Consider a sequence, which is formed by the following rule: next term is taken as the smallest possible non-negative integer, which is not yet in the sequence, so that no 3 terms of sequence form an arithmetic progression.
Example
f(0) = 0 -- smallest non-negative
f(1) = 1 -- smallest non-negative, which is not yet in the sequence
f(2) = 3 -- since 0, 1, 2 form an arithmetic progression
f(3) = 4 -- neither of 0, 1, 4, 0, 3, 4, 1, 3, 4 form an arithmetic progression, so we can take smallest non-negative, which is larger than 3
f(4) = 9 -- 5, 6, 7, 8 are not good, since 1, 3, 5, 0, 3, 6, 1, 4, 7, 0, 4, 8 are all valid arithmetic progressions.
The task
Write a function f(n), which returns the n-th member of sequence.
Limitations
There are 1000 random tests with 0 <= n <= 10^9, so you should consider algorithmic complexity of your solution.