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 dictionary for prefix sums.

DSA C#

var prefixSums = new Dictionary<int, int>[1];

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

A{1, 0}

B{0, 1}

C{0, 0}

D{1, 1}

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing with {0, 0} which misses counting subarrays starting at index 0.

Using {1, 0} or {1, 1} which are incorrect keys for prefix sums.

We start with prefix sum 0 having count 1 because the sum before starting is zero.

2fill in blank

medium

Complete the code to update the current prefix sum inside the loop.

DSA C#

currentSum [1] num;

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

A+=

B/=

C*=

D-=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-=' which subtracts instead of adding.

Using '*=' or '/=' which are not relevant for sums.

We add the current number to the running prefix sum.

3fill in blank

hard

Fix the error in checking if prefixSums contains the needed key.

DSA C#

if (prefixSums.[1](currentSum - k))

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

AContains

BContainsValue

CContainsKey

DHasKey

Attempts:

3 left

💡 Hint

Common Mistakes

Using ContainsValue which checks values, not keys.

Using Contains or HasKey which are not valid Dictionary methods.

Dictionary in C# uses ContainsKey to check if a key exists.

4fill in blank

hard

Fill both blanks to update the count of the current prefix sum in the dictionary.

DSA C#

prefixSums[[1]] = prefixSums.[2](currentSum) ? prefixSums[currentSum] + 1 : 1;

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

AcurrentSum

BContainsKey

CContainsValue

DcurrentSum - k

Attempts:

3 left

💡 Hint

Common Mistakes

Using ContainsValue instead of ContainsKey.

Using wrong key like currentSum - k instead of currentSum.

We update the count for the currentSum key. If it exists, increment; else set to 1.

5fill in blank

hard

Fill all three blanks to correctly increment the result count when a matching prefix sum is found.

DSA C#

if (prefixSums.ContainsKey([1])) result [2]= prefixSums[[3]];

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

AcurrentSum - k

B+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+=' causing wrong result.

Using wrong keys for lookup.

When prefixSums contains currentSum - k, add its count to result.

Practice

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

easy

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

B. To sort the array elements before processing

C. To store the original array elements as keys

D. To reverse the array for easier traversal

Solution

1. Step 1: Understand the problem goal

   The problem wants to find subarrays whose sums equal a target value k.

2. Step 2: Role of hash map in solution

   The hash map stores cumulative sums seen so far and how many times they occurred, enabling quick checks for needed sums.

3. Final Answer:

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

4. Quick Check:

   Hash map stores sums = A [OK]

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

Common Mistakes:

• Thinking hash map stores array elements directly
• Assuming sorting is needed
• Using hash map to reverse array

2. Which of the following is the correct way to declare a hash map (Dictionary) in C# to store cumulative sums and their counts?

easy

A. Dictionary sumCounts = new Dictionary();

B. HashMap sumCounts = new HashMap();

C. Map sumCounts = new Map();

D. Dictionary sumCounts = new Dictionary();

Solution

1. Step 1: Identify correct C# dictionary syntax

   C# uses Dictionary<TKey, TValue> with new Dictionary<TKey, TValue>() to declare hash maps.

2. Step 2: Check each option

   Dictionary sumCounts = new Dictionary(); uses correct generic syntax. Options B and C use Java-like syntax. Dictionary sumCounts = new Dictionary(); misses generic types.

3. Final Answer:

   Dictionary sumCounts = new Dictionary(); -> Option A

4. Quick Check:

   C# Dictionary syntax = A [OK]

Hint: Use Dictionary in C# for hash maps [OK]

Common Mistakes:

• Using Java HashMap syntax in C#
• Omitting generic type parameters
• Using non-existent Map type in C#

3. What is the output of the following C# code snippet?

int[] nums = {1, 2, 3};
int k = 3;
int count = 0;
Dictionary sumCounts = new Dictionary();
sumCounts[0] = 1;
int sum = 0;
foreach (int num in nums) {
    sum += num;
    if (sumCounts.ContainsKey(sum - k)) {
        count += sumCounts[sum - k];
    }
    if (sumCounts.ContainsKey(sum)) {
        sumCounts[sum]++;
    } else {
        sumCounts[sum] = 1;
    }
}
Console.WriteLine(count);

medium

A. 1

B. 0

C. 3

D. 2

Solution

1. Step 1: Calculate cumulative sums and check counts

   nums = [1,2,3], k=3. sumCounts starts with {0:1}. - sum=1: sum-k= -2 (not in sumCounts), count=0, sumCounts={0:1,1:1} - sum=3: sum-k=0 (in sumCounts=1), count=1, sumCounts={0:1,1:1,3:1} - sum=6: sum-k=3 (in sumCounts=1), count=2, sumCounts={0:1,1:1,3:1,6:1}

2. Step 2: Final count of subarrays with sum k

   Count is 2 because subarrays [1,2] and [3] sum to 3.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Subarrays count = 2 [OK]

Hint: Track cumulative sums and check sum-k in map [OK]

Common Mistakes:

• Not initializing sumCounts with {0:1}
• Forgetting to update count correctly
• Misunderstanding sum-k lookup

4. Identify the bug in this C# code snippet for counting subarrays summing to k:

int[] nums = {1, 1, 1};
int k = 2;
int count = 0;
Dictionary sumCounts = new Dictionary();
int sum = 0;
foreach (int num in nums) {
    sum += num;
    if (sumCounts.ContainsKey(sum - k)) {
        count += sumCounts[sum - k];
    }
    sumCounts[sum] = 1;
}
Console.WriteLine(count);

medium

A. nums array is empty

B. sumCounts is not initialized with sum 0 count 1

C. count is not incremented correctly

D. sum variable is not updated inside the loop

Solution

1. Step 1: Check initialization of sumCounts

   sumCounts should start with {0:1} to count subarrays starting at index 0, but here it is empty.

2. Step 2: Check updating sumCounts inside loop

   sumCounts[sum] = 1 overwrites counts instead of incrementing, but main bug is missing initial {0:1}.

3. Final Answer:

   sumCounts is not initialized with sum 0 count 1 -> Option B

4. Quick Check:

   Initialize sumCounts[0] = 1 [OK]

Hint: Always initialize sumCounts with {0:1} before loop [OK]

Common Mistakes:

• Not initializing sumCounts with {0:1}
• Overwriting counts instead of incrementing
• Ignoring sum-k lookup logic

5. Given the array nums = {3, 4, 7, 2, -3, 1, 4, 2} and k = 7, how many subarrays sum to k using the hash map approach?

hard

A. 5

B. 3

C. 4

D. 6

Solution

1. Step 1: Calculate cumulative sums and track counts

   Initialize sumCounts = {0:1}, sum=0, count=0. - i=0: sum=3, sum-k=-4(not found), count=0, sumCounts={0:1,3:1} - i=1: sum=7, sum-k=0(found=1), count=1, sumCounts={0:1,3:1,7:1} - i=2: sum=14, sum-k=7(found=1), count=2, sumCounts={0:1,3:1,7:1,14:1} - i=3: sum=16, sum-k=9(not found), count=2, sumCounts={0:1,3:1,7:1,14:1,16:1} - i=4: sum=13, sum-k=6(not found), count=2, sumCounts={0:1,3:1,7:1,14:1,16:1,13:1} - i=5: sum=14, sum-k=7(found=1), count=3, sumCounts[14]=2 - i=6: sum=18, sum-k=11(not found), count=3, sumCounts={0:1,3:1,7:1,14:2,16:1,13:1,18:1} - i=7: sum=20, sum-k=13(found=1), count=4, sumCounts={0:1,3:1,7:1,14:2,16:1,13:1,18:1,20:1}

2. Step 2: Verify the counted subarrays

   Subarrays summing to 7 are: - [3,4] - [7] - [7,2,-3,1] - [1,4,2] Count is 4 total.

3. Final Answer:

   4 -> Option C

4. Quick Check:

   Count subarrays with sum k = 4 [OK]

Hint: Track sums and counts carefully to find all subarrays [OK]

Common Mistakes:

• Missing subarrays with negative numbers
• Not updating counts when sum repeats
• Forgetting to initialize sumCounts with {0:1}