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

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