0413.go

// Source: https://leetcode.com/problems/arithmetic-slices
// Title: Arithmetic Slices
// Difficulty: Medium
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// 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
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package main

func numberOfArithmeticSlices(nums []int) int {
	n := len(nums)

	res := 0

	for i := 0; i < n-2; {
		step := nums[i+1] - nums[i]

		j := i + 1
		for ; j < n-1; j++ {
			if nums[j+1]-nums[j] != step {
				break
			}
		}

		d := j - i
		if d >= 2 {
			res += d * (d - 1) / 2
		}
		i = j
	}

	return res
}