15. 3Sum

Medium
Array Two Pointers Sorting

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.

Notice that the solution set must not contain duplicate triplets.

Example 1:

Input: nums = [-1,0,1,2,-1,-4]
Output: [[-1,-1,2],[-1,0,1]]
Explanation: 
nums[0] + nums[1] + nums[2] = (-1) + 0 + 1 = 0.
nums[1] + nums[2] + nums[4] = 0 + 1 + (-1) = 0.
nums[0] + nums[3] + nums[4] = (-1) + 2 + (-1) = 0.
The distinct triplets are [-1,0,1] and [-1,-1,2].
Notice that the order of the output and the order of the triplets does not matter.

Example 2:

Input: nums = [0,1,1]
Output: []
Explanation: The only possible triplet does not sum up to 0.

Example 3:

Input: nums = [0,0,0]
Output: [[0,0,0]]
Explanation: The only possible triplet sums up to 0.

Constraints:

  • 3 <= nums.length <= 3000
  • -10^5 <= nums[i] <= 10^5
func threeSum(nums []int) [][]int {
	h := map[int]int{}
	for _, num := range nums {
		h[num]++
	}
	// fmt.Println("H", h)
	uniq := []int{}
	for k := range h {
		uniq = append(uniq, k)
	}
	// fmt.Println("Uniq", uniq)

	result := [][]int{}
	for i := 0; i < len(uniq); i++ {
		a := uniq[i]
		for j := i; j < len(uniq); j++ {
			b := uniq[j]
			c := -a - b
			if c >= a && c >= b {
				counts := map[int]int{}
				counts[a]++
				counts[b]++
				counts[c]++

				canAdd := true
				for k, v := range counts {
					if v > h[k] {
						canAdd = false
						break
					}
				}
				if !canAdd {
					continue
				}

				result = append(result, []int{a, b, c})
			}
		}
	}

	return result
}
💡 Hints (3)
Hint 1
So, we essentially need to find three numbers x, y, and z such that they add up to the given value. If we fix one of the numbers say x, we are left with the two-sum problem at hand!
Hint 2
For the two-sum problem, if we fix one of the numbers, say x, we have to scan the entire array to find the next number y, which is value - x where value is the input parameter. Can we change our array somehow so that this search becomes faster?
Hint 3
The second train of thought for two-sum is, without changing the array, can we use additional space somehow? Like maybe a hash map to speed up the search?