public class Solution
extends Object
446 - Arithmetic Slices II - Subsequence\.
Hard
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 sequence.
A **subsequence** of an array is a sequence that can be formed by removing some elements (possibly none) of the array.
* For example, `[2,5,10]` is a subsequence of `[1,2,1, **2** ,4,1, **5** , **10** ]`.
The test cases are generated so that the answer fits in **32-bit** integer.
**Example 1:**
**Input:** nums = [2,4,6,8,10]
**Output:** 7
**Explanation:** All arithmetic subsequence slices are:
[2,4,6]
[4,6,8]
[6,8,10]
[2,4,6,8]
[4,6,8,10]
[2,4,6,8,10]
[2,6,10]
**Example 2:**
**Input:** nums = [7,7,7,7,7]
**Output:** 16
**Explanation:** Any subsequence of this array is arithmetic.
**Constraints:**
* `1 <= nums.length <= 1000`
* -231 <= nums[i] <= 231 - 1