Problem Statement in English

You’re given an integer array nums of length. Your task is to find the number of subarrays of length 3 such that the sum of their first and last element is equal to the sum of the middle element.

Approach

This is a straightforward sliding window problem.

Solution in Python


class Solution:
    def countSubarrays(self, nums: List[int]) -> int:
        res = l = 0
        s = sum(nums[:2])

        for r in range(2, len(nums)):
            s += nums[r]

            if not (nums[l + 1] % 2) and s - nums[l+1] == nums[l + 1] // 2:
                res += 1

            s -= nums[l]
            l += 1

        return res

Complexity

  • Time: $O(n)$
    Since we iterate over the array once.

  • Space: $O(1)$
    Since we only use a few variables to store the result and the current sum, the space complexity is constant.

And we are done.