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Find all unique triplets summing to zero

Quick Overview

This question evaluates array-based algorithm design, handling of duplicate results, and analysis of time and space complexity when producing unique triplets that sum to zero.

Find all unique triplets summing to zero

Company: NVIDIA

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: medium

Interview Round: Onsite

## Problem Given an integer array `nums`, return **all unique triplets** `[nums[i], nums[j], nums[k]]` such that: - `i`, `j`, and `k` are **distinct indices** (`i != j != k`), and - `nums[i] + nums[j] + nums[k] == 0`. The returned triplets must be **unique** (i.e., no duplicate triplets in the output). Triplets can be returned in any order. ## Input - `nums`: an array of integers ## Output - A list/array of unique triplets, each triplet containing three integers whose sum is zero ## Constraints (typical interview bounds) - `0 <= nums.length <= 3000` - `-10^5 <= nums[i] <= 10^5` ## Example - Input: `nums = [-1, 0, 1, 2, -1, -4]` - Output: `[[-1, -1, 2], [-1, 0, 1]]`

Quick Answer: This question evaluates array-based algorithm design, handling of duplicate results, and analysis of time and space complexity when producing unique triplets that sum to zero.

Given an integer array `nums`, return all unique triplets `[nums[i], nums[j], nums[k]]` such that `i`, `j`, and `k` are distinct indices and `nums[i] + nums[j] + nums[k] == 0`. To make the output deterministic, return each triplet in non-decreasing order, and return the full list of triplets in lexicographic order. Do not include duplicate triplets.

Constraints

0 <= len(nums) <= 3000
-100000 <= nums[i] <= 100000

Examples

Input: ([-1, 0, 1, 2, -1, -4],)

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

Explanation: The only unique triplets that sum to zero are [-1, -1, 2] and [-1, 0, 1].

Input: ([0, 0, 0, 0],)

Expected Output: [[0, 0, 0]]

Explanation: Even though there are multiple ways to choose three zeros, they all form the same triplet, so it appears once.

Input: ([],)

Expected Output: []

Explanation: An empty array cannot contain any triplet.

Input: ([1, 2, -2, -1],)

Expected Output: []

Explanation: No combination of three numbers sums to zero.

Input: ([-2, 0, 1, 1, 2],)

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

Explanation: The two unique triplets summing to zero are [-2, 0, 2] and [-2, 1, 1].

Hints

Sorting the array helps you avoid duplicates and makes it possible to use a two-pointer scan.
Fix one number, then look for two remaining numbers whose sum equals its negation.

Last updated: May 18, 2026

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