Hi, I recently came across this question and i have no idea how to approach it.
You are given a number N (3 ≤ N ≤ 100000) and you should create a permutation of the first N numbers.
The permutation should not have an arithmetic subsequence with three or more terms in it.
for example
5 4 2 3 1 5 3 3 1 2
Take a look at this problem (and its solution). http://codeforces.net/contest/452/problem/F