# 0446. Arithmetic Slices II - Subsequence

https://leetcode.com/problems/arithmetic-slices-ii-subsequence

## Description

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`

## ac

```java
```
