FIND Arithmetic Slices II - Subsequence - Leetcode 446 #dynamicprogramming

Leetcode 446. Arithmetic Slices II Subsequence - Python Solution

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.
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Key Moments
0:00 Intro to how to find Arithmetic Slices II - Subsequence
0:19 Explanation of How to Find Arithmetic Slices II - Subsequence
7:36 Code for How to Find Arithmetic Slices II - Subsequence
8:40 Results
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[Digifan]

At 6:27 why we take the difference 2 from the last indexes and not also from 4?