Four Sum Problem All Unique Quadruplets in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sort the input array before processing.

DSA Javascript

nums.[1]((a, b) => a - b);

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

Asort

Bmap

Cfilter

Dreverse

Attempts:

3 left

💡 Hint

Common Mistakes

Using reverse instead of sort

Trying to filter before sorting

Sorting the array helps to efficiently find quadruplets and avoid duplicates.

2fill in blank

medium

Complete the code to skip duplicate values for the first pointer i.

DSA Javascript

if (i > 0 && nums[i] === nums[i - 1]) [1];

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

Areturn

Bbreak

Ccontinue

Dthrow

Attempts:

3 left

💡 Hint

Common Mistakes

Using break which exits the loop

Using return which exits the function

Using 'continue' skips the current iteration to avoid duplicate quadruplets starting with the same number.

3fill in blank

hard

Fix the error in the while loop condition to move the left pointer correctly.

DSA Javascript

while (left < right && nums[left] === nums[left [1] 1]) left++;

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

A/

B+

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using + instead of - causing out of range

Using * or / which are invalid here

To check the previous element for duplicates, subtract 1 from left index.

4fill in blank

hard

Fill both blanks to correctly move the right pointer and skip duplicates.

DSA Javascript

while (left < right && nums[right] === nums[right [1] 1]) right[2];

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

A-

B+

C--

D++

Attempts:

3 left

💡 Hint

Common Mistakes

Using - instead of + for comparison

Using ++ which moves pointer wrong way

We compare nums[right] with nums[right + 1] to skip duplicates (the position just passed when moving leftward), and decrement right pointer with --.

5fill in blank

hard

Fill all three blanks to add the found quadruplet and move both pointers.

DSA Javascript

result.push([nums[i], nums[j], nums[left], nums[right]]); left[1]; right[2]; while (left < right && nums[left] === nums[left [3] 1]) left++;

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

A++

B--

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong pointer movements

Not skipping duplicates correctly

After adding quadruplet, move left pointer forward with ++, right pointer backward with --, then skip duplicates by comparing with previous element using subtraction.

Practice

1. What is the main goal of the Four Sum Problem in data structures and algorithms?

easy

A. Find all unique quadruplets in an array that sum up to a target value

B. Sort the array in ascending order

C. Find the maximum element in the array

D. Count the number of pairs with a given sum

Solution

1. Step 1: Understand the problem statement

   The Four Sum Problem asks us to find all unique groups of four numbers in an array that add up to a specific target sum.

2. Step 2: Differentiate from other problems

   This is different from sorting, finding max, or counting pairs because it specifically looks for quadruplets with a sum equal to the target.

3. Final Answer:

   Find all unique quadruplets in an array that sum up to a target value -> Option A

4. Quick Check:

   Four Sum Problem = Find quadruplets summing to target [OK]

Hint: Look for four numbers adding to target, not sorting or max [OK]

Common Mistakes:

• Confusing with sorting or max element problems
• Looking for pairs instead of quadruplets
• Ignoring uniqueness of quadruplets

2. Which of the following is the correct way to start sorting an array nums in JavaScript before solving the Four Sum Problem?

easy

A. nums.sort()

B. nums.sort((a, b) => b - a)

C. nums.sorted()

D. nums.sort((a, b) => a - b)

Solution

1. Step 1: Understand JavaScript sort behavior

   By default, nums.sort() sorts elements as strings, which can give wrong order for numbers.

2. Step 2: Use numeric comparison function

   Using nums.sort((a, b) => a - b) sorts numbers in ascending order correctly, which is needed for the Four Sum algorithm.

3. Final Answer:

   nums.sort((a, b) => a - b) -> Option D

4. Quick Check:

   Numeric ascending sort = nums.sort((a, b) => a - b) [OK]

Hint: Use numeric compare function for sorting numbers [OK]

Common Mistakes:

• Using default sort without compare function
• Using descending order when ascending is needed
• Using non-existent method like sorted()

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

const nums = [1, 0, -1, 0, -2, 2];
const target = 0;
// Assume findFourSum returns all unique quadruplets summing to target
const result = findFourSum(nums, target);
console.log(result);

Given findFourSum correctly implements the Four Sum algorithm, what is the expected output?

medium

A. [[-2, -1, 1, 2], [-2, 0, 0, 2], [-1, 0, 0, 1]]

B. [[1, 0, -1, 0], [-2, 2, 0, 0]]

C. [[-2, -1, 0, 1], [-1, 0, 0, 1]]

D. [[0, 0, 0, 0]]

Solution

1. Step 1: Sort the array and find quadruplets

   Sorted array is [-2, -1, 0, 0, 1, 2]. The unique quadruplets summing to 0 are [-2, -1, 1, 2], [-2, 0, 0, 2], and [-1, 0, 0, 1].

2. Step 2: Verify quadruplets sum to target

   Each quadruplet sums to 0, matching the target, and all are unique sets.

3. Final Answer:

   [[-2, -1, 1, 2], [-2, 0, 0, 2], [-1, 0, 0, 1]] -> Option A

4. Quick Check:

   Unique quadruplets summing to 0 = [[-2, -1, 1, 2], [-2, 0, 0, 2], [-1, 0, 0, 1]] [OK]

Hint: Sort array first, then find quadruplets summing to target [OK]

Common Mistakes:

• Not sorting array before processing
• Including duplicate quadruplets
• Incorrect quadruplet sums

4. Identify the error in the following snippet for solving the Four Sum Problem:

function fourSum(nums, target) {
  nums.sort();
  const quadruplets = [];
  for (let i = 0; i < nums.length - 3; i++) {
    for (let j = i + 1; j < nums.length - 2; j++) {
      let left = j + 1;
      let right = nums.length - 1;
      while (left < right) {
        const sum = nums[i] + nums[j] + nums[left] + nums[right];
        if (sum === target) {
          quadruplets.push([nums[i], nums[j], nums[left], nums[right]]);
          left++;
          right--;
        } else if (sum < target) {
          left++;
        } else {
          right--;
        }
      }
    }
  }
  return quadruplets;
}

medium

A. The loops do not cover all indices correctly

B. The array is not sorted numerically before processing

C. The quadruplets array is not returned

D. The sum calculation is incorrect

Solution

1. Step 1: Check sorting method

   The code uses nums.sort() without a compare function, which sorts elements as strings, causing incorrect order for numbers.

2. Step 2: Impact on algorithm correctness

   Incorrect sorting leads to wrong quadruplets or missed solutions because the two-pointer approach depends on sorted numeric order.

3. Final Answer:

   The array is not sorted numerically before processing -> Option B

4. Quick Check:

   Missing numeric sort compare function = The array is not sorted numerically before processing [OK]

Hint: Always use numeric compare in sort for number arrays [OK]

Common Mistakes:

• Using default sort without compare function
• Not returning the result array
• Incorrect loop ranges (not here though)
• Wrong sum calculation (correct here)

5. Given an array nums = [2, 2, 2, 2, 2] and target = 8, what is the correct output of the Four Sum Problem algorithm that finds all unique quadruplets?

hard

A. [[2, 2, 2]]

B. [[2, 2, 2, 2], [2, 2, 2, 2]]

C. [[2, 2, 2, 2]]

D. []

Solution

1. Step 1: Understand the input and target

   All elements are 2, and target is 8. Four 2's sum to 8 exactly.

2. Step 2: Identify unique quadruplets

   Even though there are multiple 2's, only one unique quadruplet [2, 2, 2, 2] exists that sums to 8.

3. Final Answer:

   [[2, 2, 2, 2]] -> Option C

4. Quick Check:

   Unique quadruplet summing to 8 = [[2, 2, 2, 2]] [OK]

Hint: Duplicates form one unique quadruplet if sum matches target [OK]

Common Mistakes:

• Listing duplicate quadruplets multiple times
• Returning empty when quadruplet exists
• Confusing triplets with quadruplets