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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the map for prefix sums.

DSA Go

prefixSums := make(map[int]int)
prefixSums[[1]] = 1

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

A1

B0

C-1

Dk

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing prefixSums with 1 instead of 0

Using k as the initial key

Starting with negative one as key

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 running sum with the current number.

DSA Go

for _, num := range nums {
    sum [1]= num
    // rest of code
}

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

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition

Using multiplication or division

We add the current number to the running sum to keep track of the prefix sum.

3fill in blank

hard

Fix the error in checking if (sum - k) exists in the map.

DSA Go

if count, ok := prefixSums[sum [1] k]; ok {
    result += count
}

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

Using multiplication or division

We check if (sum - k) exists because it means a subarray sums to k.

4fill in blank

hard

Fill both blanks to update the map with the current sum count.

DSA Go

prefixSums[sum] = prefixSums[[1]] [2] 1

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

Asum

B+

C-

Dk

Attempts:

3 left

💡 Hint

Common Mistakes

Using k as key instead of sum

Subtracting 1 instead of adding

We increase the count of the current prefix sum by 1 in the map.

5fill in blank

hard

Fill all three blanks to complete the function that returns the count of subarrays summing to k.

DSA Go

func subarraySum(nums []int, k int) int {
    prefixSums := make(map[int]int)
    prefixSums[[1]] = 1
    sum := 0
    result := 0
    for _, num := range nums {
        sum [2]= num
        if count, ok := prefixSums[sum [3] k]; ok {
            result += count
        }
        prefixSums[sum] = prefixSums[sum] + 1
    }
    return result
}

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

A0

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Wrong initialization of prefixSums

Using wrong operators in sum update or map lookup

The function initializes prefixSums with 0 key, adds current num to sum, checks if (sum - k) exists, and updates result accordingly.

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 store cumulative sums and their frequencies for quick lookup

D. To keep track of the maximum subarray length

Solution

1. Step 1: Understand the problem goal

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

2. Step 2: Role of hash map in solution

   A hash map stores cumulative sums seen so far and their counts, enabling quick checks if a previous sum exists that forms a subarray summing to k.

3. Final Answer:

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

4. Quick Check:

   Hash map = cumulative sums storage [OK]

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

Common Mistakes:

• Thinking hash map stores array elements as keys
• Assuming sorting is needed
• Confusing hash map with array for sums

2. Which of the following Go code snippets correctly initializes a hash map to store integer keys and values for the subarray sum problem?

easy

A. m := make([]int, 0)

B. m := make(map[int]int)

C. m := map[int]string{}

D. m := map[string]int{}

Solution

1. Step 1: Identify correct map type for keys and values

   We need a map with integer keys and integer values to store sums and counts.

2. Step 2: Check Go syntax for map initialization

   make(map[int]int) correctly creates a map with int keys and int values.

3. Final Answer:

   m := make(map[int]int) -> Option B

4. Quick Check:

   map[int]int = correct map type [OK]

Hint: Use make(map[int]int) for int keys and values [OK]

Common Mistakes:

• Using slice instead of map
• Wrong key or value types in map
• Using map with string keys incorrectly

3. What is the output of the following Go code snippet?

nums := []int{1, 2, 3}
k := 3
count := 0
sum := 0
m := make(map[int]int)
m[0] = 1
for _, v := range nums {
    sum += v
    if val, ok := m[sum-k]; ok {
        count += val
    }
    m[sum]++
}
fmt.Println(count)

medium

A. 1

B. 0

C. 3

D. 2

Solution

1. Step 1: Trace cumulative sums and map updates

   Iteration 1: sum=1, m[1]=1, sum-k=-2 not in map, count=0 Iteration 2: sum=3, sum-k=0 in map with count 1, count=1, m[3]=1 Iteration 3: sum=6, sum-k=3 in map with count 1, count=2, m[6]=1

2. Step 2: Final count after loop

   Count is 2 because two subarrays sum to k=3: [1,2] and [3].

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Subarrays summing to 3 = 2 [OK]

Hint: Count increments when (sum-k) found in map [OK]

Common Mistakes:

• Not initializing m[0] = 1
• Forgetting to increment map count
• Miscounting subarrays

4. Identify the bug in the following Go code snippet for counting subarrays summing to k:

nums := []int{1, 1, 1}
k := 2
count := 0
sum := 0
m := make(map[int]int)
for _, v := range nums {
    sum += v
    if val, ok := m[sum-k]; ok {
        count += val
    }
    m[sum]++
}
fmt.Println(count)

medium

A. Missing initialization m[0] = 1 before loop

B. Incorrect increment of count variable

C. Wrong loop variable used

D. Using map with wrong key type

Solution

1. Step 1: Check map initialization

   The map m is used to store cumulative sums, but m[0] is not initialized to 1 before the loop.

2. Step 2: Effect of missing m[0] = 1

   Without m[0] = 1, subarrays starting from index 0 that sum to k are not counted, causing incorrect count.

3. Final Answer:

   Missing initialization m[0] = 1 before loop -> Option A

4. Quick Check:

   Initialize m[0]=1 to count subarrays from start [OK]

Hint: Always set m[0] = 1 before loop [OK]

Common Mistakes:

• Not initializing m[0]
• Incrementing count incorrectly
• Using wrong map key type

5. Given the array nums = []int{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 count.

hard

A. 4

B. 3

C. 6

D. 5

Solution

1. Step 1: Calculate cumulative sums and track counts

   Initialize m[0]=1, sum=0, count=0 Iterate nums: - sum=3, sum-k=-4 not in m, m[3]=1 - sum=7, sum-k=0 in m=1, count=1, m[7]=1 - sum=14, sum-k=7 in m=1, count=2, m[14]=1 - sum=16, sum-k=9 not in m, m[16]=1 - sum=13, sum-k=6 not in m, m[13]=1 - sum=14, sum-k=7 in m=1, count=3, m[14]=2 - sum=18, sum-k=11 not in m, m[18]=1 - sum=20, sum-k=13 in m=1, count=4, m[20]=1

2. Step 2: Verify subarrays

   The subarrays summing to 7 are: [3,4], [7], [7,2,-3,1], [1,4,2]. Total count = 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Count subarrays with sum k = 4 [OK]

Hint: Track cumulative sums and counts carefully [OK]

Common Mistakes:

• Missing m[0] initialization
• Miscounting overlapping subarrays
• Not updating map counts correctly