Count Number of Nice Subarrays

Given an array of integers nums and an integer k. A continuous subarray is called nice if there are k odd numbers on it.

Return the number of nice sub-arrays.

Example 1:

Input: nums = [1,1,2,1,1], k = 3
Output: 2
Explanation: The only sub-arrays with 3 odd numbers are [1,1,2,1] and [1,2,1,1].

Example 2:

Input: nums = [2,4,6], k = 1
Output: 0
Explanation: There is no odd numbers in the array.

Example 3:

Input: nums = [2,2,2,1,2,2,1,2,2,2], k = 2
Output: 16

Constraints:

• 1 <= nums.length <= 50000
• 1 <= nums[i] <= 10^5
• 1 <= k <= nums.length

SOLUTION:

class Solution:
    def numberOfSubarrays(self, nums: List[int], k: int) -> int:
        n = len(nums)
        ctr = 0
        odd = 0
        prefix = [0] * (n + 1)
        for i in range(n):
            prefix[odd] += 1
            if nums[i] & 1:
                odd += 1
            if odd >= k:
                ctr += prefix[odd - k]
        return ctr