Recursion - recursive functions - swift.

RecursiveFunctions.playground

/*
Things to 'remember' about recursion:

--- you can call a function inside of itself
--- you always have at least "ONE BASE CASE" in order to prevent the function calling itself infinite times.
*/

func fibonacci(_ i: Int) -> Int {
    if i <= 2 {
        return 1
    } else {
        return fibonacci(i - 1) + fibonacci(i - 2)
    }
}

func factorial(_ N: Int) -> Int {
    if N == 1 {
        return 1
    } else {
        return N * factorial(N - 1)
    }
}

func digits(_ number:Int) -> [Int] {
    if number >= 10 {
        let firstDigit = digits(number / 10)
        let lastDigit = number % 10
        return firstDigit + [lastDigit]
    } else {
        return [number]
    }
}

func pow(_ x: Int, _ y: Int) -> Int {
    
    if y == 0 {
        return 1
    } else {
        return x * pow(x, y - 1)
    }
}


func euclidian(_ a: Int, _ b: Int) -> Int {
    
    if b == 0 {
        return a
    } else {
        if a > b {
            return euclidian(a - b, b)
        } else {
            return euclidian(a, b - a)
        }
    }
}


func binarySearch(_ key:Int, _ numbers:[Int], left:Int = 0, right:Int = -1) -> Bool {
    
    var right = right
    if right == -1 {
        right = numbers.count - 1
    }
    
    if left < right {
        print(#line, "left:", left, "-right:", right)
         var mid = (left + right) / 2
        print(#line, "mid: ", mid)
        if key < numbers[mid] {
            return binarySearch(key, numbers, left: left, right: mid)
        } else if key > numbers[mid] {
            return binarySearch(key, numbers, left: mid + 1, right: right)
        } else {
            return true
        }
        
    } else {
        return numbers[left] == key
    }
    
}