<p>Given an array of integers <code>nums</code> and an integer <code>k</code>. A continuous subarray is called <strong>nice</strong> if there are <code>k</code> odd numbers on it.</p>

<p>Return <em>the number of <strong>nice</strong> sub-arrays</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

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

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nums = [2,4,6], k = 1
<strong>Output:</strong> 0
<strong>Explanation:</strong> There are no odd numbers in the array.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> nums = [2,2,2,1,2,2,1,2,2,2], k = 2
<strong>Output:</strong> 16
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 50000</code></li>
	<li><code>1 &lt;= nums[i] &lt;= 10^5</code></li>
	<li><code>1 &lt;= k &lt;= nums.length</code></li>
</ul>

from collections import defaultdict

class Solution:
    def numberOfSubarrays(self, nums: List[int], k: int) -> int:
        prefix_sum = defaultdict(int)
        prefix_sum[0] = 1
        count = 0
        total = 0
        for num in nums:
            if num % 2 == 1:
                total += 1
            prefix_sum[total] += 1
            if (total - k) in prefix_sum:
                count += prefix_sum[total - k]
        return count

        # prefix_sum = dict()
        # prefix_sum[0] = 1
        # sum = 0
        # result = 0
        # for num in nums:
        #     if num % 2 == 1:
        #         sum += 1
        #     if sum - k in prefix_sum:
        #         result += prefix_sum[sum - k]
        #     if sum not in prefix_sum:
        #         prefix_sum[sum] = 0
        #     prefix_sum[sum] += 1
        # return result
23100