recursive binary search

//
//  main.swift
//  exAlgorithms
//
//  Created by Judar Lima on 13/01/19.
//  Copyright © 2019 Judar Lima. All rights reserved.
//

import Foundation

func binarySearch(by number: Int, at list: [Int]) -> String {
    let left = list.startIndex
    let right = list.endIndex - 1
    let errorMessage = "The number \(number) does not exist in this list"
    let numberFound = "The number \(number) is in the list at index: "
    let result = divideAndCoquer(with: number, list: list, left: left, right: right)

    return result == nil ? errorMessage : numberFound + "\(String(describing: result!))"
}

func divideAndCoquer(with number: Int, list: [Int], left: Int, right: Int) -> Int? {
    let middle = (left + right) / 2
    if number < list[left] || number > list[right] {
        return nil
    }
    else if number == list[left] {
        return left
    }
    else if number == list[right] {
        return right
    }
    else if number == list[middle] {
        return middle
    }
    else if number < list[middle] {
        return divideAndCoquer(with: number, list: list, left: left, right: middle)
    }
    else if number > list[middle] {
        return divideAndCoquer(with: number, list: list, left: middle, right: right)
    }
    return nil
}
let list = [1, 3, 6, 7, 8, 10, 22, 23, 27, 29, 30, 33, 35, 38, 41, 43, 44, 46]
let randomNumber = list.randomElement()
print(binarySearch(by: randomNumber!, at: list))
print("\t _________________________________")

let invalidNumber = 9123
print(binarySearch(by: invalidNumber, at: list))