<p>Given a binary array <code>nums</code> and an integer <code>goal</code>, return <em>the number of non-empty <strong>subarrays</strong> with a sum</em> <code>goal</code>.</p>

<p>A <strong>subarray</strong> is a contiguous part of the array.</p>

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

<pre>
<strong>Input:</strong> nums = [1,0,1,0,1], goal = 2
<strong>Output:</strong> 4
<strong>Explanation:</strong> The 4 subarrays are bolded and underlined below:
[<u><strong>1,0,1</strong></u>,0,1]
[<u><strong>1,0,1,0</strong></u>,1]
[1,<u><strong>0,1,0,1</strong></u>]
[1,0,<u><strong>1,0,1</strong></u>]
</pre>

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

<pre>
<strong>Input:</strong> nums = [0,0,0,0,0], goal = 0
<strong>Output:</strong> 15
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 3 * 10<sup>4</sup></code></li>
	<li><code>nums[i]</code> is either <code>0</code> or <code>1</code>.</li>
	<li><code>0 &lt;= goal &lt;= nums.length</code></li>
</ul>

class Solution:
    def numSubarraysWithSum(self, nums: List[int], goal: int) -> int:
        l = 0
        r = 0
        windowSum = 0
        ans = 0

        while r < len(nums):
            if l == r:
                windowSum += nums[r]
                r += 1

            # initialize
            while windowSum < goal and r < len(nums):
                windowSum += nums[r]
                r += 1
            
            if windowSum < goal:
                break
            
            if windowSum > goal:
                windowSum -= nums[l]
                l += 1
                continue

            ans += 1
            trailingZeros = 0
            # print(l, r, ans, trailingZeros, nums[l:r])
            
            # extend
            while r < len(nums) and nums[r] == 0:
                ans += 1
                r += 1
                trailingZeros += 1

            # print(l, r, ans, trailingZeros, nums[l:r])

            while l < r-1 and nums[l] == 0:
                if r-l == trailingZeros + 1:
                    trailingZeros -= 1
                ans += trailingZeros + 1

                l += 1
                # print(l, r, ans, trailingZeros, nums[l:r])
                
            windowSum -= nums[l]
            l += 1

        return ans
17500