LeetCode #15 3Sum

LeetCode #15 3Sum

Question

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],
A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

Solution

Approach #1

class Solution {
    func threeSum(_ nums: [Int]) -> [[Int]] {
        if nums.count < 3 { return [] }
        var sortedNums = nums.sorted()
        var results = [[Int]]()
        for i in 0..<sortedNums.count - 2 {
            if i == 0 || sortedNums[i] != sortedNums[i - 1] {
                var l = i + 1
                var r = sortedNums.count - 1
                let target = -sortedNums[i]
                while l < r {
                    let sum = sortedNums[l] + sortedNums[r]
                    if sum == target {
                        results.append([sortedNums[i], sortedNums[l], sortedNums[r]])
                        while l < r, sortedNums[l] == sortedNums[l + 1] { l += 1 }
                        l += 1
                        while l < r, sortedNums[r] == sortedNums[r - 1] { r -= 1 }
                        r -= 1
                    } else if sum > target {
                        while l < r, sortedNums[r] == sortedNums[r - 1] { r -= 1 }
                        r -= 1
                    } else {
                        while l < r, sortedNums[l] == sortedNums[l + 1] { l += 1 }
                        l += 1
                    }
                }
            }
        }
        return results
    }
}

Time complexity: O(n^2).

Space complexity: O(n). Sorted array, which has the same length of original array, needs space.

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