413. Arithmetic Slices ⭐

Medium

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]
Output:
 3
Explanation:
 We have 3 arithmetic slices in nums: [1, 2, 3], [2, 3, 4] and [1,2,3,4] itself.

Example 2:

Input: nums = [1]
Output:
 0

Constraints:

• 1 <= nums.length <= 5000

• -1000 <= nums[i] <= 1000

解題

使用 DP 的概念 來記錄。

func numberOfArithmeticSlices(nums []int) int {
    dp := make([]int, len(nums))
    res := 0

    for i:=2; i<len(nums); i++ {
        if nums[i] - nums[i-1] == nums[i-1] - nums[i-2] {
            dp[i] = dp[i-1] + 1
        }
        res += dp[i]
    }

    return res
}