Problem List

3Sum

May 30, 2025 • Go • brute force, hashmap • medium

Problem

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

Approach

1. Sort the array first to make scanning easier and to ensure triplets are ordered consistently for deduplication.
2. Use a map to store all indices of each number. For every unique pair (i, j), compute the required third number to reach 0, and look it up in the index map.
3. Ensure the third index is different from i and j.
4. Sort each triplet before storing it, and use a map with a string key to avoid storing duplicates.

Edge Cases

All zeros: if the set has only 0 and length >= 3, return a single triplet [0,0,0] early.
Multiple duplicate triplets: handled by using string deduplication map.
Negative and positive values interleaved: sorting the array upfront helps ensure order when forming string keys.

Reflections

This was an exercise in edge-case handling. The brute-force double loop actually isn't too bad given the problem constraints, especially once the deduplication logic was cleaned up. Switching from O(n) equality checks with slices to using a map of string keys for O(1) lookup time came after the initial pass.

While not the optimal solution in terms of runtime, this version is readable, covers all edge cases, and was a fun way to grind out performance. The initial set check in the beginning is to cover the first edge case mentioned.

Performance

Runtime beats: 7%
Memory beats: 14%

Complexity

Time: O(n^2 * log n) Iterate over all pairs (i, j), then use a hash map lookup for the third number. The result deduplication uses string keys, so constant insertion/checks, but overall loop is O(n^2). Sorting each triplet is O(log 3) = O(1).
Space: O(n^2)Hash map for number indices and a set for deduplicating triplets. In worst case, could store up to O(n^2) unique triplets.

Go Solution

func threeSum(nums []int) [][]int {
	set := toSet(nums)
	tripletSets := make(map[string]bool)

	for num := range set {
		if num == 0 && len(set) == 1 && len(nums) >= 3 {
			res = append(res, []int{num, num, num})
			return res
		}
		break
	}

	var res [][]int
	sort.Ints(nums)


	hash := make(map[int][]int)
	for i, num := range nums {
		hash[num] = append(hash[num], i)
	}

	for i, num1 := range nums {
		for j := i + 1; j < len(nums); j++ {
			num2 := nums[j]

			target := -1 * (num1 + num2)
			if indexes, ok := hash[target]; ok {
				idx := findCorrectIndex(i, j, indexes)
				if idx == -1 {
					continue
				}
				arr := []int{num1, num2, nums[idx]}
				sort.Ints(arr)
				key := fmt.Sprintf("%d%d%d", arr[0], arr[1], arr[2])

				if _, seen := tripletSets[key]; !seen {
					tripletSets[key] = true
					res = append(res, arr)
				}
			}
		}
	}

	return res
}

func findCorrectIndex(i, j int, indexes []int) int {
	for _, k := range indexes {
		if i != j && i != k && j != k {
			return k
		}
	}
	return -1
}

func toSet(nums []int) map[int]struct{} {
	Set := make(map[int]struct{})
	for _, num := range nums {
		Set[num] = struct{}{}
	}
	return Set
}

LeetCode Problem Link