446. Arithmetic Slices II - Subsequence

Given an integer array nums, return the number of all the arithmetic subsequences of nums. A sequence of numbers is called arithmetic if it consists of at least three elements and if the difference between any two consecutive elements is the same. - For example, [1, 3, 5, 7, 9], [7, 7, 7, 7], and [3, -1, -5, -9] are arithmetic sequences. - For example, [1, 1, 2, 5, 7] is not an arithmetic seque