Longest Arithmetic Progression

Problem Setter: dhruvi
Problem Tester: sneh_gupta

Find longest Arithmetic Progression in an integer array and return its length. More formally, find longest sequence of indeces, 0 < i1 < i2 < … < ik < ArraySize(0-indexed) such that sequence A[i1], A[i2], …, A[ik] is an Arithmetic Progression. Arithmetic Progression is a sequence in which all the differences between consecutive pairs are the same, i.e sequence B[0], B[1], B[2], …, B[m - 1] of length m is an Arithmetic Progression if and only if B[1] - B[0] == B[2] - B[1] == B[3] - B[2] == … == B[m - 1] - B[m - 2].
Examples
1) 1, 2, 3(All differences are equal to 1)
2) 7, 7, 7(All differences are equal to 0)
3) 8, 5, 2(Yes, difference can be negative too)

Samples
1) Input: 3, 6, 9, 12
Output: 4

2) Input: 9, 4, 7, 2, 10
Output: 3(If we choose elements in positions 1, 2 and 4(0-indexed))

NOTE: You only need to implement the given function. Do not read input, instead use the arguments to the function. Do not print the output, instead return values as specified. Still have a doubt? Checkout Sample Codes for more details.
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