How should I practice Count Subarrays with Equal Even and Odd Counts?

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Count Subarrays with Equal Even and Odd Counts

Microsoft● MediumINTERNONSITE INTERVIEW

Problem statement

You are given an integer array nums. A subarray is balanced when it contains as many even values as odd values.

Return the number of balanced subarrays. A subarray is a contiguous, non-empty part of the array. Subarrays that start or end at different indices are counted separately, even when they hold the same values.

Function

countBalancedSubarrays(nums: int[]) → long

Examples

Example 1

nums = [3,8,5,6]return = 4

The numbers alternate between odd and even. The balanced subarrays are [3, 8], [8, 5], [5, 6] and the whole array [3, 8, 5, 6], which has two even and two odd numbers. A subarray of odd length can never be balanced, and these four are all the subarrays of even length, so the answer is 4.

Example 2

nums = [4,10,2]return = 0

All three numbers are even, so every subarray has more even numbers than odd numbers. No subarray is balanced.

Example 3

nums = [7,0,2,9,9]return = 4

0 is even. The balanced subarrays are [7, 0], [2, 9], [0, 2, 9, 9] and [7, 0, 2, 9]. For example, [0, 2, 9, 9] has the even numbers 0 and 2 and the odd numbers 9 and 9.

Constraints

1 <= nums.length <= 10^5
0 <= nums[i] <= 10^9