Question

Given an array a of integers, return the length of the longest arithmetic subsequence in a.

Answer 1

Answer:

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 arithmeti