LeetCode #16 3Sum Closest

LeetCode #16 3Sum Closest

Question

Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

For example, given array S = {-1 2 1 -4}, and target = 1.
The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

Solution

Approach #1

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

Time complexity: O(n^2).

Space complexity: O(n).

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