Subarray Sum Equals K Using Hash Map in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of subarray sum count with negative numbers

What is the output of the following code that counts subarrays summing to k using a hash map?

DSA Python

def subarray_sum(nums, k):
    count = 0
    current_sum = 0
    sum_map = {0: 1}
    for num in nums:
        current_sum += num
        if current_sum - k in sum_map:
            count += sum_map[current_sum - k]
        sum_map[current_sum] = sum_map.get(current_sum, 0) + 1
    return count

print(subarray_sum([1, -1, 0], 0))

A 3

B 2

C 1

D 0

💡 Hint

Think about all subarrays that sum to zero, including those with negative numbers.

The subarrays that sum to 0 are: [1, -1], [-1, 0], and [0]. So total count is 3.

❓ Predict Output

intermediate

Count subarrays summing to k with repeated elements

What does this code print when counting subarrays summing to 3?

DSA Python

def subarray_sum(nums, k):
    count = 0
    current_sum = 0
    sum_map = {0: 1}
    for num in nums:
        current_sum += num
        if current_sum - k in sum_map:
            count += sum_map[current_sum - k]
        sum_map[current_sum] = sum_map.get(current_sum, 0) + 1
    return count

print(subarray_sum([1, 2, 1, 2, 1], 3))

A 4

B 3

C 5

D 2

💡 Hint

Count all subarrays that sum exactly to 3, including overlapping ones.

The subarrays summing to 3 are: [1,2], [2,1], [1,2], [2,1]. Total 4.

🔧 Debug

advanced

Identify the error in subarray sum code

What error does this code raise when run?

DSA Python

def subarray_sum(nums, k):
    count = 0
    current_sum = 0
    sum_map = {}
    for num in nums:
        current_sum += num
        if current_sum - k in sum_map:
            count += sum_map[current_sum - k]
        sum_map[current_sum] = sum_map.get(current_sum, 0) + 1
    return count

print(subarray_sum([1, 2, 3], 3))

A 0

B KeyError

C 3

D TypeError

💡 Hint

Check if the initial sum_map contains the prefix sum 0.

sum_map is empty initially, so checking current_sum - k for 0 fails with KeyError.

❓ Predict Output

advanced

Output of subarray sum count with zeros and k=0

What is the output of this code counting subarrays summing to 0?

DSA Python

def subarray_sum(nums, k):
    count = 0
    current_sum = 0
    sum_map = {0: 1}
    for num in nums:
        current_sum += num
        if current_sum - k in sum_map:
            count += sum_map[current_sum - k]
        sum_map[current_sum] = sum_map.get(current_sum, 0) + 1
    return count

print(subarray_sum([0, 0, 0], 0))

A 1

B 3

C 6

D 0

💡 Hint

Count all subarrays of zeros that sum to zero.

All subarrays of zeros sum to zero: [0], [0], [0], [0,0], [0,0], [0,0,0] total 6.

🧠 Conceptual

expert

Why use a hash map for subarray sum equals k?

Which reason best explains why a hash map is used in the subarray sum equals k problem?

A To reverse the array for easier traversal

B To sort the array elements for binary search

C To store all subarrays explicitly for counting

D To store counts of prefix sums for quick lookup of needed sums

💡 Hint

Think about how prefix sums help find subarrays quickly.

The hash map stores counts of prefix sums to quickly find how many previous sums match current_sum - k.