# Leetcode 413 - Arithmetic Slices

`Note:`

- Subarray/subsequence problems are usually dp.
`dp[i]`

means the`longest`

arithmetic progression ending with nums[i].- When
`nums[i-2], nums[i-1], nums[i]`

forms an arithmetic array,`dp[i] = dp[i-1] + 1`

. - If not,
`dp[i] = 2`

, and we add`dp[i-1]`

to the ans[] to calculate the last`longest length`

of arithmetic array. - Finally, use math
`(n - 3 + 1)*(n - 3 + 2) / 2`

to calculate the num of subarrays when the arithmetic array’s length is`n`

.

`Question:`

An integer array 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.

Given an integer array `nums`

, return the `number`

of arithmetic subarrays of nums.

A subarray is a contiguous subsequence of the array.

`Example:`

1 | Input: nums = [1,2,3,4] |

`Code:`

1 | /** |