Subarray Sum Equals K Using Hash Map in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the variable that stores the count of subarrays.

DSA C

int count = [1];

Drag options to blanks, or click blank then click option'

A1

B0

C-1

Dk

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing count to 1 or negative values.

We start counting subarrays from zero, so initializing count to 0 is correct.

2fill in blank

medium

Complete the code to update the cumulative sum with the current array element.

DSA C

sum [1]= nums[i];

Drag options to blanks, or click blank then click option'

A-

B/

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition.

We add the current element to the cumulative sum to keep track of the total so far.

3fill in blank

hard

Fix the error in the condition that checks if (sum - k) exists in the hash map.

DSA C

if (hashMapContains(hashMap, sum [1] k)) {

Drag options to blanks, or click blank then click option'

A-

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or multiplication instead of subtraction.

We check if sum - k exists because it indicates a subarray summing to k.

4fill in blank

hard

Fill both blanks to update the count and the hash map correctly.

DSA C

count += hashMapGet(hashMap, sum [1] k);
hashMapPut(hashMap, sum, hashMapGet(hashMap, sum) [2] 1);

Drag options to blanks, or click blank then click option'

A-

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of subtraction in first blank or wrong operator in second blank.

We add the frequency of (sum - k) to count and increment the frequency of sum by 1.

5fill in blank

hard

Fill all three blanks to initialize the hash map, insert the base case, and iterate over the array.

DSA C

HashMap *hashMap = [1]();
hashMapPut(hashMap, 0, [2]);
for (int i = 0; i [3] n; i++) {

Drag options to blanks, or click blank then click option'

AhashMapCreate

B1

C<

DhashMapInit

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong function name, wrong base frequency (like 0), or wrong loop condition.

We create the hash map, set the base case frequency of sum 0 to 1, and loop while i is less than n.

Practice

1. What is the main purpose of using a hash map in the Subarray Sum Equals K problem?

easy

A. To store the original array elements as keys

B. To sort the array elements before processing

C. To keep track of the maximum subarray length

D. To store cumulative sums and their frequencies for quick lookup

Solution

1. Step 1: Understand the problem requirement

   The problem requires finding subarrays whose sums equal a target value k.

2. Step 2: Role of hash map in the solution

   The hash map stores cumulative sums seen so far and their counts to quickly check if a subarray sum equals k exists.

3. Final Answer:

   To store cumulative sums and their frequencies for quick lookup -> Option D

4. Quick Check:

   Hash map stores sums for fast subarray sum checks [OK]

Hint: Hash map tracks sums to find subarrays fast [OK]

Common Mistakes:

• Thinking hash map sorts elements
• Using hash map to store array elements directly
• Confusing hash map with array for indexing

2. Which of the following is the correct way to initialize a hash map in C for storing integer keys and values?

easy

A. map map;

B. HashMap map = new HashMap<>();

C. int map[1000] = {0};

D. unordered_map map;

Solution

1. Step 1: Identify C language syntax

   In C, hash maps are not built-in; arrays can be used as simple maps for small ranges.

2. Step 2: Check options for C syntax

   int map[1000] = {0}; uses an integer array initialized to zero, suitable as a simple map in C.

3. Final Answer:

   int map[1000] = {0}; -> Option C

4. Quick Check:

   Array initialization is valid C syntax [OK]

Hint: Use arrays for maps in C, no built-in hash map [OK]

Common Mistakes:

• Using C++ or Java syntax in C code
• Trying to declare hash maps directly in C
• Not initializing arrays before use

3. What will be the output of the following code snippet in C that counts subarrays summing to k=5?

int arr[] = {1, 2, 3, 2, 1};
int k = 5;
// Assume correct hash map logic implemented
int result = subarraySum(arr, 5, k);
printf("%d", result);

medium

A. 3

B. 2

C. 1

D. 0

Solution

1. Step 1: Identify subarrays summing to 5

   Subarrays: [2,3] (indices 1-2), [3,2] (indices 2-3), [2,3] (indices 3-4); total 3 subarrays.

2. Step 2: Count valid subarrays

   Counting carefully, the subarrays summing to 5 are three in total.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Count of subarrays with sum 5 = 3 [OK]

Hint: Count all subarrays with sum k using prefix sums [OK]

Common Mistakes:

• Counting only non-overlapping subarrays
• Missing subarrays starting at index 0
• Incorrectly summing elements

4. Identify the bug in this C code snippet for counting subarrays with sum k using a hash map:

int subarraySum(int* nums, int numsSize, int k) {
    int count = 0, sum = 0;
    int map[10000] = {0};
    map[0] = 1;
    for (int i = 0; i < numsSize; i++) {
        sum += nums[i];
        if (map[sum - k])
            count += map[sum - k];
        map[sum]++;
    }
    return count;
}

medium

A. Array map size is too small for negative sums

B. Missing initialization of count variable

C. Incorrect loop range causing out-of-bounds access

D. Using map as pointer without allocation

Solution

1. Step 1: Analyze map usage for sums

   The map array uses indices as sums, but sums can be negative, causing invalid indexing.

2. Step 2: Identify impact of negative sums

   Negative sums cause out-of-bound access or wrong counts since array indices can't be negative.

3. Final Answer:

   Array map size is too small for negative sums -> Option A

4. Quick Check:

   Negative sums need offset or different data structure [OK]

Hint: Handle negative sums with offset or hash map [OK]

Common Mistakes:

• Ignoring negative sums in indexing
• Assuming sums are always positive
• Not initializing map for sum 0

5. Given an array arr = {3, 4, 7, 2, -3, 1, 4, 2} and k = 7, how many subarrays sum to k? Use the hash map approach to find the answer.

hard

A. 4

B. 5

C. 3

D. 6

Solution

1. Step 1: Calculate prefix sums and track counts

   Prefix sums: 3,7,14,16,13,14,18,20. Check sums - k in map to count subarrays.

2. Step 2: Count subarrays with sum k=7

   Subarrays: [3,4], [7], [7,2,-3,1], [1,4,2], [4,2, -3,1,4] total 5 subarrays.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Count subarrays summing to 7 = 5 [OK]

Hint: Use prefix sums and hash map to count subarrays fast [OK]

Common Mistakes:

• Missing subarrays with negative numbers
• Counting overlapping subarrays incorrectly
• Not updating hash map counts properly