Skip to main content

15. 3Sum

mediumAsked at Klarna

Find all unique triplets in an array that sum to zero.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, j != k, and the three values sum to zero. The result must not contain duplicate triplets.

Constraints

  • 3 <= nums.length <= 3000
  • -10^5 <= nums[i] <= 10^5

Examples

Example 1

Input
nums = [-1,0,1,2,-1,-4]
Output
[[-1,-1,2],[-1,0,1]]

Example 2

Input
nums = [0,1,1]
Output
[]

Approaches

1. Triple loop

Enumerate every triple and use a set of sorted keys to dedupe.

Time
O(n^3)
Space
O(n)
function threeSum(nums) {
  const out = new Set();
  for (let i = 0; i < nums.length; i++)
    for (let j = i+1; j < nums.length; j++)
      for (let k = j+1; k < nums.length; k++)
        if (nums[i]+nums[j]+nums[k] === 0)
          out.add([nums[i],nums[j],nums[k]].sort((a,b)=>a-b).join(','));
  return [...out].map(s => s.split(',').map(Number));
}

Tradeoff:

2. Sort + two-pointer

Sort the array, then for each i, run a two-pointer sweep across the remainder looking for the pair that completes a zero sum. Skip duplicates as you go.

Time
O(n^2)
Space
O(1)
function threeSum(nums) {
  nums.sort((a, b) => a - b);
  const out = [];
  for (let i = 0; i < nums.length - 2; i++) {
    if (i > 0 && nums[i] === nums[i-1]) continue;
    let l = i + 1, r = nums.length - 1;
    while (l < r) {
      const s = nums[i] + nums[l] + nums[r];
      if (s === 0) {
        out.push([nums[i], nums[l], nums[r]]);
        while (l < r && nums[l] === nums[l+1]) l++;
        while (l < r && nums[r] === nums[r-1]) r--;
        l++; r--;
      } else if (s < 0) l++;
      else r--;
    }
  }
  return out;
}

Tradeoff:

Klarna-specific tips

Klarna interviewers care more about how cleanly you skip duplicate triplets than about the asymptotic — sloppy dedupe is what they see in production risk-feature jobs that explode under cardinality.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill 3Sum and other Klarna interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →