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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sort the input list before processing.

DSA Python

nums = [1, 0, -1, 0, -2, 2]
nums.[1]()

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Asort

Breverse

Cappend

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using reverse() instead of sort()

Trying to append or pop elements here

Sorting the list 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 Python

for i in range(len(nums) - 3):
    if i > 0 and nums[i] == nums[[1]]:
        continue

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A0

Bi + 1

Ci - 1

Dlen(nums)

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with i + 1 which is the next element

Comparing with 0 index always

To skip duplicates, compare current element with the previous one at i-1.

3fill in blank

hard

Fix the error in the while loop condition to avoid infinite loops.

DSA Python

while left < right:
    total = nums[i] + nums[j] + nums[left] + nums[right]
    if total == target:
        result.append([nums[i], nums[j], nums[left], nums[right]])
        left += 1
        right -= 1
        while left < right and nums[left] == nums[[1]]:
            left += 1

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Aleft - 1

Bleft + 1

Cright + 1

Dright - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using left + 1 which causes skipping too far

Using right pointers incorrectly

We compare nums[left] with the previous left value at left - 1 to skip duplicates.

4fill in blank

hard

Fill both blanks to skip duplicates for the second pointer j.

DSA Python

for j in range(i + 1, len(nums) - 2):
    if j > i + 1 and nums[j] == nums[[1]]:
        continue
    left, right = j + 1, len(nums) - 1
    while left < right:
        # code to find quadruplets
        if total == target:
            # after adding quadruplet
            while left < right and nums[right] == nums[[2]]:
                right -= 1

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Aj - 1

Bj + 1

Cright + 1

Dright - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using j + 1 instead of j - 1 for skipping duplicates

Using right + 1 instead of right - 1 for skipping duplicates

To skip duplicates for j, compare with previous j-1; for right pointer, compare with right-1.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that counts quadruplets by their sum.

DSA Python

quadruplet_sums = { [1]: [2] for quad in quadruplets if sum(quad) [3] target }

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Asum(quad)

Bquad

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '==' in the condition

Swapping key and value in the dictionary comprehension

We create a dictionary with sum(quad) as key and quad as value, filtering sums equal to target.