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: A = [3,6,9,12] Output: 4 Explanation: The whole array is an arithmetic sequence with steps of length = 3. Example 2: Input: A = [9,4,7,2,10] Output: 3 Explanation: The longest arithmetic subsequence is [4,7,10]. Example 3: Input: A = [20,1,15,3,10,5,8] Output: 4 Explanation: The longest arithmetic subsequence is [20,15,10,5]. Constraints: 2 <= A.length <= 1000 0 <= A[i] <= 500