Given an array A of integers, return the length of the longest
arithmetic subsequence in A.
Recall that a subsequence of A is a list A[i_1], A[i_2], ...,
A[i_k] with 0 <= i_1 < i_2 < ... < i_k <= A.length - 1,
and that a sequence B is arithmetic if B[i+1] - B[i]
are all the same value (for 0 <= i < B.length - 1).
Example 1:
Input: [3,6,9,12] Output: 4 Explanation: The whole array is an arithmetic sequence with steps of length = 3.
Example 2:
Input: [9,4,7,2,10] Output: 3 Explanation: The longest arithmetic subsequence is [4,7,10].
Example 3:
Input: [20,1,15,3,10,5,8] Output: 4 Explanation: The longest arithmetic subsequence is [20,15,10,5].