Sync LeetCode submission Runtime - 28 ms (70.47%), Memory - 21.5 MB (5.54%)

Author: jimit105

0966-binary-subarrays-with-sum/README.md

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+<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>

0966-binary-subarrays-with-sum/solution.py

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+# Approach 1: Prefix Sum
+
+# Time: O(n)
+# Space: O(n)
+
+class Solution:
+    def numSubarraysWithSum(self, nums: List[int], goal: int) -> int:
+        total_count = 0
+        curr_sum = 0
+        sum_freq = {0 : 1} # {prefix_sum : frequency}
+
+        for num in nums:
+            curr_sum += num
+            if curr_sum - goal in sum_freq:
+                total_count += sum_freq[curr_sum - goal]
+            
+            sum_freq[curr_sum] = sum_freq.get(curr_sum, 0) + 1
+
+        return total_count
+
+
+