Three Sum Problem All Unique Triplets in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal of the Three Sum Problem?

To find all unique triplets in an array that sum up to zero.

beginner

Why do we sort the array before solving the Three Sum Problem?

Sorting helps to avoid duplicates easily and allows using two pointers to find pairs efficiently.

intermediate

Explain the two-pointer technique used in the Three Sum Problem.

After fixing one element, use two pointers starting from the next element and the end of the array to find pairs that sum with the fixed element to zero.

intermediate

How do you avoid duplicate triplets in the Three Sum Problem?

Skip duplicate elements when moving the fixed pointer and also skip duplicates when moving the two pointers.

intermediate

What is the time complexity of the optimal Three Sum solution?

O(n^2), where n is the number of elements in the array.

What is the first step in solving the Three Sum Problem?

A Use a hash map

B Reverse the array

C Sort the array

D Find the maximum element

Sorting the array helps in efficiently finding triplets and avoiding duplicates.

In the two-pointer approach, after fixing one element, where do the two pointers start?

A Both at the start of the array

B One just after the fixed element, one at the end of the array

C Both at the end of the array

D Random positions

One pointer starts just after the fixed element, and the other starts at the end to find pairs summing to the target.

How do you handle duplicates when the fixed element is the same as the previous one?

A Skip it to avoid duplicate triplets

B Include it anyway

C Double the count

D Remove all duplicates from the array

Skipping duplicates prevents repeated triplets in the result.

What is the sum target for the pairs when the fixed element is nums[i]?

A nums[i]

B 0

C nums[i] * 2

D -nums[i]

The pairs must sum to the negative of the fixed element to make the total zero.

What is the overall time complexity of the optimal Three Sum solution?

A O(n^2)

B O(n log n)

C O(n^3)

D O(n)

The solution uses sorting (O(n log n)) and a nested loop with two pointers (O(n^2)), so overall O(n^2).

Describe the step-by-step approach to solve the Three Sum Problem with unique triplets.

Think about sorting, fixing one number, and two-pointer search.

Explain how to avoid duplicate triplets in the Three Sum Problem solution.

Duplicates appear when same numbers are repeated; skipping them avoids repeated triplets.