swift solution to codility demo task

demotask.swift

// MissingInteger
// Find the smallest positive integer that does not occur in a given sequence. 

//This is a demo task.
//
//Write a function:
//
//public func solution(_ A : inout [Int]) -> Int
//
//that, given an array A of N integers, returns the smallest positive integer (greater than 0) that does not occur in A.
//
//For example, given A = [1, 3, 6, 4, 1, 2], the function should return 5.
//
//Given A = [1, 2, 3], the function should return 4.
//
//Given A = [−1, −3], the function should return 1.
//
//Write an efficient algorithm for the following assumptions:
//
//N is an integer within the range [1..100,000];
//each element of array A is an integer within the range [−1,000,000..1,000,000].
//

let A = [1, 3, 6, 4, 1, 2]
let A2 =  [1, 2, 3]
let A3 = [-1, -3]

public func solution(_ a : [Int]) -> Int {
    let noNegative = a.filter { $0 > 0 }
    let uniqueNumbers = Set(noNegative)
    var uniqueArray = Array(uniqueNumbers)
    uniqueArray.sort()
    for (index, number) in uniqueArray.enumerated() {
        if number > index + 1 {
            return index + 1
        }
    }
    let largestIntNotPresent = (uniqueArray.last ?? 0) + 1
    return largestIntNotPresent
}

let a = solution(A)
let a2 = solution(A2)
let a3 = solution(A3)