https://leetcode.com/problems/arithmetic-slices/

413_leetcode.py

class Solution:
    # sliding window(O(n))
    def numberOfArithmeticSlices(self, nums: List[int]) -> int:
        def numberOfArithmeticArr(n: int) -> int:
            if n < 3:
                return 0
            return ((n - 2) * (n - 1)) // 2
        
        if len(nums) < 3:
            return 0
        
        startIdx = 0
        endIdx = 1
        diff = nums[startIdx] - nums[endIdx]
        result = 0
        for i in range(2, len(nums)):
            
            diff2 = nums[endIdx] - nums[i]
            
            if diff != diff2:
                # end of arithmetic slice
                if endIdx - startIdx > 1:
                    result += numberOfArithmeticArr(n = endIdx - startIdx + 1)
                startIdx = endIdx
                endIdx = i
                diff = nums[startIdx] - nums[endIdx]
            else:
                endIdx = i
        
        if endIdx - startIdx > 1:
            result += numberOfArithmeticArr(n = endIdx - startIdx + 1)
        
        return result