3392. Count Subarrays of Length Three With a Condition

Given an integer array nums, return the number of subarrays of length 3 such that the sum of the first and third numbers equals exactly half of the second number.

Example 1:

Input: nums = [1,2,1,4,1]

Output: 1

Explanation:

Only the subarray [1,4,1] contains exactly 3 elements where the sum of the first and third numbers equals half the middle number.

Example 2:

Input: nums = [1,1,1]

Output: 0

Explanation:

[1,1,1] is the only subarray of length 3. However, its first and third numbers do not add to half the middle number.

def countSubarrays(self, nums: List[int]) -> int:
	if len(nums) < 3:
		return 0
	count = 0
	for i in range(len(nums)-2):
		if (nums[i] + nums[i+2]) * 2 == nums[i+1]:
			count += 1
	return count