Skip to main content

17. 3Sum

mediumAsked at Grab

Find all unique triplets that sum to zero — Grab uses this as a sort-plus-two-pointer fluency check.

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

Problem

Given an integer array nums, return all unique triplets [nums[i], nums[j], nums[k]] such that i, j, k are distinct and nums[i] + nums[j] + nums[k] == 0.

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. Brute force triple loop

Try every triple; dedupe via sorted-tuple Set.

Time
O(n^3)
Space
O(n)
const seen = new Set();
for (let i = 0; i < n; i++)
  for (let j = i + 1; j < n; j++)
    for (let k = j + 1; k < n; k++)
      if (nums[i] + nums[j] + nums[k] === 0) seen.add(...);

Tradeoff:

2. Sort and two-pointer

Sort the array; for each pivot i, shrink l/r toward each other to find pairs summing to -nums[i].

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:

Grab-specific tips

Grab interviewers want explicit dedupe logic — frame as finding three drivers whose ride balances net to zero across the SEA wallet.

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 Grab interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →