Tesla Interview — Find All Unique Triplets Summing to Target | 3Sum Generalized Version | Two-Pointer Optimization

Given an array of integers nums and an integer target, return an array of arrays containing all unique triplets that add up to target.

Each returned triplet must be in ascending order, and the final result must not contain duplicate triplets.

You may assume nums has at least 3 elements.

Example:

Input:
nums = [-1, 0, 1, 2, -1, -4]
target = 0

Output:
[[-1, -1, 2], [-1, 0, 1]]

This is a generalized version of the classic 3Sum problem.

The optimal strategy:

1. Sort the array
2. Fix the first element using index i
3. Use a two-pointer approach (l and r) to find pairs that sum to target − nums[i]
4. Carefully skip duplicates for:
   • the fixed index i
   • the left pointer (l)
   • the right pointer (r)

Time complexity is O(n²); sorting helps deduplicate easily and ensures the triplets are sorted.

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