Pair of Sum. Θ(nlgn)

Describe a Θ(nlgn) time algorithm that, given a set S of n integers and another integer x, determines whether or not there exist two elements in S whose sum is exactly x.

func pairOf(sum:Int, in array:inout [Int]) throws -> (Int, Int)? {
    guard array.count>1 else { throw TestError.argumentFault }
    mergeSort(array: &array)
    var low = 0, high = array.count-1
    while low != high {
        var currentSum = array[low] + array[high]
        if currentSum > sum { high -= 1 }
        else if currentSum < sum { low += 1 }
        else { return (array[low], array[high]) }
    }
    return nil
}

var s = try Int.randomArray()
let sum = try Int.random()
try pairOf(sum: sum, in: &s)