Find the longest arithmetic series that can be formed by choosing a sub-collection (possibly the entire collection)..TC problem

An arithmetic series consists of a sequence of terms such that each term minus its immediate predecessor gives the same result. For example, the sequence 3,7,11,15 is the terms of the arithmetic series 3+7+11+15; each term minus its predecessor equals 4. (Of course there is no requirement on the first term since it has no predecessor.)

Given a collection of integers, we want to find the longest arithmetic series that can be formed by choosing a sub-collection (possibly the entire collection). Create a class ASeries that contains a method longest that is given a int[] values and returns the length of the longest arithmetic series that can be formed from values.

Definition

Class: ASeries
Method: longest
Parameters: int[]
Returns: int
Method signature:int longest(int[] values) (be sure your method is public)

Constraints
- values will contain between 2 and 50 elements inclusive.
- Each element of values will be between -1,000,000 and 1,000,000 inclusive.

Examples
0)



{3,8,4,5,6,2,2}

Returns: 5

No arithmetic series using these values is longer than 2,3,4,5,6.
1)



{-1,-5,1,3}

Returns: 3

-1, 1, 3 is an arithmetic series (so is 3,-1,-5).
2)



{-10,-20,-10,-10}

Returns: 3

-10,-10,-10 is an arithmetic series.

I Took The Problem From My Friend Rizwan's Blog (he is Kind oF Computer Geek) :)
he told that success rate of this problem is about 52% so that makes me crazy to,solve it.

first let me try myself thinking how i can approach