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14. 3Sum

mediumAsked at Postman

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

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

Problem

Given an integer array nums, return all unique triplets [a, b, c] such that a + b + c = 0 and the indices are distinct. 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 = [0,0,0]
Output
[[0,0,0]]

Approaches

1. Triple loop

Enumerate every triple and use a Set of sorted-string keys to dedupe.

Time
O(n^3)
Space
O(n^2)
// nested i,j,k; if sum==0 add JSON.stringify([a,b,c].sort()) to set

Tradeoff:

2. Sort + two-pointer

Sort the array; for each fixed i, walk lo and hi pointers inward, skipping duplicates on all three positions.

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 lo = i+1, hi = nums.length - 1;
    while (lo < hi) {
      const s = nums[i] + nums[lo] + nums[hi];
      if (s === 0) {
        out.push([nums[i], nums[lo], nums[hi]]);
        while (lo < hi && nums[lo] === nums[lo+1]) lo++;
        while (lo < hi && nums[hi] === nums[hi-1]) hi--;
        lo++; hi--;
      } else if (s < 0) lo++;
      else hi--;
    }
  }
  return out;
}

Tradeoff:

Postman-specific tips

Postman expects the dedupe-skip pattern verbalized — it's the same shape as deduping repeated headers in a request before signing it.

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Output

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