[Swift 4.2] Update Karatsuba multiplication

Changed the way the characters in a String were accessed since `characters` was deprecated on Swift 4.0.
Removed the code snippet at the beggining of the file.

Author: afonsograca

Karatsuba Multiplication/KaratsubaMultiplication.playground/Contents.swift

@@ -1,10 +1,7 @@
 //: Playground - noun: a place where people can play
 
 import Foundation
-// last checked with Xcode 9.0b4
-#if swift(>=4.0)
-print("Hello, Swift 4")
-#endif
+
 precedencegroup ExponentiativePrecedence {
   higherThan: MultiplicationPrecedence
   lowerThan: BitwiseShiftPrecedence
@@ -18,8 +15,8 @@ func ^^ (radix: Int, power: Int) -> Int {
 
 // Long Multiplication - O(n^2)
 func multiply(_ num1: Int, by num2: Int, base: Int = 10) -> Int {
-  let num1Array = String(num1).characters.reversed().map { Int(String($0))! }
-  let num2Array = String(num2).characters.reversed().map { Int(String($0))! }
+  let num1Array = String(num1).reversed().map { Int(String($0))! }
+  let num2Array = String(num2).reversed().map { Int(String($0))! }
 
   var product = Array(repeating: 0, count: num1Array.count + num2Array.count)
 
@@ -38,14 +35,14 @@ func multiply(_ num1: Int, by num2: Int, base: Int = 10) -> Int {
 
 // Karatsuba Multiplication - O(n^log2(3))
 func karatsuba(_ num1: Int, by num2: Int) -> Int {
-  let num1Array = String(num1).characters
-  let num2Array = String(num2).characters
+  let num1String = String(num1)
+  let num2String = String(num2)
 
-  guard num1Array.count > 1 && num2Array.count > 1 else {
+  guard num1String.count > 1 && num2String.count > 1 else {
     return multiply(num1, by: num2)
   }
 
-  let n = max(num1Array.count, num2Array.count)
+  let n = max(num1String.count, num2String.count)
   let nBy2 = n / 2
 
   let a = num1 / 10^^nBy2

Karatsuba Multiplication/KaratsubaMultiplication.playground/playground.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>IDEDidComputeMac32BitWarning</key>
+	<true/>
+</dict>
+</plist>

Karatsuba Multiplication/KaratsubaMultiplication.swift

@@ -20,14 +20,14 @@ func ^^ (radix: Int, power: Int) -> Int {
 }
 
 func karatsuba(_ num1: Int, by num2: Int) -> Int {
-  let num1Array = String(num1).characters
-  let num2Array = String(num2).characters
+  let num1String = String(num1)
+  let num2String = String(num2)
 
-  guard num1Array.count > 1 && num2Array.count > 1 else {
-    return num1 * num2
+  guard num1String.count > 1 && num2String.count > 1 else {
+    return multiply(num1, by: num2)
   }
-  
-  let n = max(num1Array.count, num2Array.count)
+
+  let n = max(num1String.count, num2String.count)
   let nBy2 = n / 2
 
   let a = num1 / 10^^nBy2