Recall & Review
beginner
What is the main idea behind using a hash map to find subarrays with sum equal to k?
We store the cumulative sums and their frequencies in a hash map to quickly find if a subarray with sum k exists by checking if (current_sum - k) has appeared before.
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beginner
Why do we use cumulative sums in the Subarray Sum Equals K problem?
Cumulative sums help us find the sum of any subarray by subtracting two cumulative sums, making it easier to check if a subarray sums to k.
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intermediate
In the hash map approach, what does the key and value represent?
The key is a cumulative sum value, and the value is the count of how many times this cumulative sum has appeared so far.
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intermediate
What is the time complexity of the Subarray Sum Equals K algorithm using a hash map?
The time complexity is O(n) because we traverse the array once and perform constant time hash map operations.
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intermediate
How does the algorithm handle the case when a subarray starts from index 0?
We initialize the hash map with {0:1} to count the empty subarray sum, so if cumulative sum equals k at any point, it counts as a valid subarray starting from index 0.
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What does the hash map store in the Subarray Sum Equals K problem?
✗ Incorrect
The hash map stores cumulative sums as keys and their frequencies as values to quickly find subarrays summing to k.
Why do we check if (current_sum - k) exists in the hash map?
✗ Incorrect
If (current_sum - k) exists, it means a subarray summing to k ends at the current index.
What initial value do we put in the hash map before starting the iteration?
✗ Incorrect
We initialize with {0:1} to handle subarrays starting at index 0.
What is the time complexity of the hash map solution for Subarray Sum Equals K?
✗ Incorrect
The algorithm runs in O(n) time by iterating once and using constant time hash map operations.
What does the value in the hash map represent?
✗ Incorrect
The value is the count of how many times a cumulative sum has appeared.
Explain how the hash map helps find the number of subarrays that sum to k.
Think about how cumulative sums and their counts relate to subarray sums.
You got /4 concepts.
Describe the steps to implement the Subarray Sum Equals K algorithm using a hash map.
Focus on initialization, iteration, and hash map usage.
You got /5 concepts.