Lonely Integer.swift

import Foundation

/*
 * Complete the 'lonelyinteger' function below.
 *
 * The function is expected to return an INTEGER.
 * The function accepts INTEGER_ARRAY a as parameter.
 */

func lonelyinteger(a: [Int]) -> Int {
    var numbers: [Int] = a
    var result: Int = numbers.removeFirst()
    numbers.forEach {
        result = result ^ $0
    }

    return result
}

let stdout = ProcessInfo.processInfo.environment["OUTPUT_PATH"]!
FileManager.default.createFile(atPath: stdout, contents: nil, attributes: nil)
let fileHandle = FileHandle(forWritingAtPath: stdout)!

guard let n = Int((readLine()?.trimmingCharacters(in: .whitespacesAndNewlines))!)
else { fatalError("Bad input") }

guard let aTemp = readLine()?.replacingOccurrences(of: "\\s+$", with: "", options: .regularExpression) else { fatalError("Bad input") }

let a: [Int] = aTemp.split(separator: " ").map {
    if let aItem = Int($0) {
        return aItem
    } else { fatalError("Bad input") }
}

guard a.count == n else { fatalError("Bad input") }

let result = lonelyinteger(a: a)

fileHandle.write(String(result).data(using: .utf8)!)
fileHandle.write("\n".data(using: .utf8)!)