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

Practice - 5 Tasks

1fill in blank

easy

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

DSA Javascript

let count = [1];

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

A1

B0

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing count to 1 instead of 0

Using null or undefined for count

We start counting subarrays from zero, so count should be initialized to 0.

2fill in blank

medium

Complete the code to update the cumulative sum with the current number in the array.

DSA Javascript

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 instead of addition

Using multiplication or division

We add the current number to the cumulative sum to keep track of the sum of elements so far.

3fill in blank

hard

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

DSA Javascript

if (map.has(sum [1] k)) {

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

A+

B/

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using sum + k instead of sum - k

Using multiplication or division

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

4fill in blank

hard

Fill both blanks to update the count and map correctly inside the loop.

DSA Javascript

count [1]= map.get(sum - k) || 0;
map.set(sum, (map.get(sum) || 0) [2] 1);

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 number of times (sum - k) occurred to count, and increment the count of current sum in map by 1.

5fill in blank

hard

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

DSA Javascript

function subarraySum(nums, k) {
  let count = [1];
  let sum = 0;
  let map = new Map();
  map.set(0, [2]);
  for (let i = 0; i < nums.length; i++) {
    sum [3]= nums[i];
    if (map.has(sum - k)) {
      count += map.get(sum - k);
    }
    map.set(sum, (map.get(sum) || 0) + 1);
  }
  return count;
}

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

A0

B1

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing count to 1

Setting map.set(0, 0) instead of 1

Using '-' instead of '+=' for sum update

Initialize count to 0, set the frequency of prefix sum 0 to 1 (to handle subarrays from index 0), and update the cumulative sum using '+='.

Practice

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

easy

A. To reverse the array for easier traversal

B. To sort the array elements before processing

C. To store the original array elements as keys

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

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 and their counts to quickly check 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 D

4. Quick Check:

   Hash map stores sums = D [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
• Ignoring cumulative sums concept

2. Which of the following is the correct syntax to initialize a JavaScript Map to store cumulative sums in the Subarray Sum Equals K problem?

easy

A. const map = new Map();

B. const map = Map();

C. const map = {};

D. const map = new map();

Solution

1. Step 1: Identify correct Map initialization syntax

   In JavaScript, a Map is created using new Map() constructor.

2. Step 2: Check options for correct syntax

   const map = {}; uses object literal {} which is not a Map. const map = Map();: Map() misses new. const map = new Map();: new Map() is correct. const map = new map();: new map() uses invalid lowercase constructor.

3. Final Answer:

   const map = new Map(); -> Option A

4. Quick Check:

   Map initialization = new Map() [OK]

Hint: Use 'new Map()' to create a Map in JavaScript [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using object literal instead of Map
• Incorrect capitalization of Map

3. What will be the output of the following code snippet?

function subarraySum(nums, k) {
  let count = 0, sum = 0;
  const map = new Map();
  map.set(0, 1);
  for (let num of nums) {
    sum += num;
    if (map.has(sum - k)) {
      count += map.get(sum - k);
    }
    map.set(sum, (map.get(sum) || 0) + 1);
  }
  return count;
}

console.log(subarraySum([1, 2, 3], 3));

medium

A. 3

B. 1

C. 2

D. 0

Solution

1. Step 1: Trace cumulative sums and map updates

   nums = [1,2,3], k=3
   sum=0 initially, map={0:1}
   Iteration 1: sum=1, sum-k= -2 (not in map), map={0:1,1:1}
   Iteration 2: sum=3, sum-k=0 (in map with count 1), count=1, map={0:1,1:1,3:1}
   Iteration 3: sum=6, sum-k=3 (in map with count 1), count=2, map={0:1,1:1,3:1,6:1}

2. Step 2: Calculate final count

   Count of subarrays summing to 3 is 2: [1,2] and [3].

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Subarrays with sum 3 = 2 [OK]

Hint: Track sums and counts in map to find subarrays fast [OK]

Common Mistakes:

• Not initializing map with (0,1)
• Forgetting to add counts from map
• Incorrect sum-k calculation

4. Identify the bug in the following code for Subarray Sum Equals K and choose the correct fix.

function subarraySum(nums, k) {
  let count = 0, sum = 0;
  const map = new Map();
  for (let num of nums) {
    sum += num;
    if (map.has(sum - k)) {
      count += map.get(sum - k);
    }
    map.set(sum, (map.get(sum) || 0) + 1);
  }
  return count;
}

medium

A. Change map.set to map.add inside loop

B. Initialize map with map.set(0, 1) before loop

C. Remove the if condition checking map.has(sum - k)

D. Initialize count to 1 instead of 0

Solution

1. Step 1: Identify missing initialization

   The map must start with sum 0 having count 1 to handle subarrays starting at index 0.

2. Step 2: Fix by adding map.set(0, 1)

   Adding map.set(0, 1) before the loop ensures correct counting of subarrays.

3. Final Answer:

   Initialize map with map.set(0, 1) before loop -> Option B

4. Quick Check:

   Start map with (0,1) fixes bug [OK]

Hint: Always initialize map with (0,1) before loop [OK]

Common Mistakes:

• Not initializing map with (0,1)
• Using map.add instead of map.set
• Removing essential if condition

5. Given the array nums = [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 cumulative sums and track counts

   Initialize map with {0:1}, sum=0, count=0
   3: sum=3, sum-k=-4 (not), map={0:1,3:1}
   4: sum=7, 0 (1), count=1, map+={7:1}
   7: sum=14, 7 (1), count=2, map+={14:1}
   2: sum=16, 9 (not), map+={16:1}
   -3: sum=13, 6 (not), map+={13:1}
   1: sum=14, 7 (1), count=3, map 14:2
   4: sum=18, 11 (not), map+={18:1}
   2: sum=20, 13 (1), count=4, map+={20:1}

2. Step 2: Identify all subarrays summing to k

   Subarrays: [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 7 = 4 [OK]

Hint: Count repeated sums in map for total subarrays [OK]

Common Mistakes:

• Ignoring repeated cumulative sums
• Not initializing map with (0,1)
• Miscounting subarrays overlapping