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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Subarray Sum Count

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

DSA C

#include <stdio.h>
#include <stdlib.h>

int subarraySum(int* nums, int numsSize, int k) {
    int count = 0, sum = 0;
    int *hash = calloc(2001, sizeof(int));
    hash[1000] = 1; // offset for sum=0
    for (int i = 0; i < numsSize; i++) {
        sum += nums[i];
        if (sum - k + 1000 >= 0 && sum - k + 1000 <= 2000) {
            count += hash[sum - k + 1000];
        }
        hash[sum + 1000]++;
    }
    free(hash);
    return count;
}

int main() {
    int nums[] = {1, 2, 3};
    int k = 3;
    printf("%d\n", subarraySum(nums, 3, k));
    return 0;
}

A 0

B 1

C 2

D 3

💡 Hint

Think about all subarrays that sum to 3: [1,2] and [3].

The subarrays that sum to 3 are [1,2] and [3], so the count is 2.

❓ Predict Output

intermediate

Result of Subarray Sum Count with Negative Numbers

What is the output of this C code counting subarrays summing to k with negative numbers?

DSA C

#include <stdio.h>
#include <stdlib.h>

int subarraySum(int* nums, int numsSize, int k) {
    int count = 0, sum = 0;
    int *hash = calloc(4001, sizeof(int));
    hash[2000] = 1; // offset for sum=0
    for (int i = 0; i < numsSize; i++) {
        sum += nums[i];
        if (sum - k + 2000 >= 0 && sum - k + 2000 <= 4000) {
            count += hash[sum - k + 2000];
        }
        hash[sum + 2000]++;
    }
    free(hash);
    return count;
}

int main() {
    int nums[] = {1, -1, 0};
    int k = 0;
    printf("%d\n", subarraySum(nums, 3, k));
    return 0;
}

A 1

B 3

C 2

D 0

💡 Hint

Count all subarrays that sum to zero including single elements and combinations.

Subarrays summing to 0 are: [1,-1], [-1,0], [0], so total 3.

🔧 Debug

advanced

Identify the Runtime Error in Subarray Sum Code

Which option will cause a runtime error when running this subarray sum code?

DSA C

#include <stdio.h>
#include <stdlib.h>

int subarraySum(int* nums, int numsSize, int k) {
    int count = 0, sum = 0;
    int *hash = calloc(1000, sizeof(int));
    hash[500] = 1;
    for (int i = 0; i < numsSize; i++) {
        sum += nums[i];
        count += hash[sum - k + 500];
        hash[sum + 500]++;
    }
    free(hash);
    return count;
}

int main() {
    int nums[] = {1, 2, 3};
    int k = 3;
    printf("%d\n", subarraySum(nums, 3, k));
    return 0;
}

A Accessing hash at negative index causes runtime error

B Memory leak error

C Division by zero error occurs

D No runtime error, code runs fine

💡 Hint

Check array index calculations for negative values.

sum-k+500 can be negative causing invalid array access and runtime error.

🧠 Conceptual

advanced

Why Use a Hash Map for Subarray Sum Equals K?

Why is a hash map used in the subarray sum equals k problem instead of nested loops?

A To reduce time complexity from O(n^2) to O(n) by storing prefix sums counts

B To save memory by storing only unique elements

C To sort the array before processing

D To avoid using recursion

💡 Hint

Think about how prefix sums and counts help find subarrays quickly.

Hash map stores counts of prefix sums to find subarrays in linear time.

🚀 Application

expert

Find Number of Subarrays Summing to k in Large Array

Given an array of 10,000 elements all equal to 1 and k = 3, what is the number of subarrays that sum to k?

A 9999

B 9997

C 10000

D 9998

💡 Hint

Count how many continuous subarrays of length 3 exist in the array.

Each subarray of length 3 sums to 3. Number of such subarrays = n - k + 1 = 10000 - 3 + 1 = 9998.