Added playground demonstrating radixSort.

Author: taiheng

Radix Sort/RadixSort.playground/Contents.swift

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+//: Playground - noun: a place where people can play
+
+import Cocoa
+
+// Test Radix Sort with small array of ten values
+var array: [Int] = [19, 4242, 2, 9, 912, 101, 55, 67, 89, 32]
+radixSort(&array)
+
+// Test Radix Sort with large array of 1000 random values
+var bigArray = [Int](repeating: 0, count: 1000)
+var i = 0
+while i < 1000 {
+  bigArray[i] = Int(arc4random_uniform(1000) + 1)
+  i += 1
+}
+radixSort(&bigArray)

Radix Sort/RadixSort.playground/Sources/radixSort.swift

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+/*
+ 
+ Sorting Algorithm that sorts an input array of integers digit by digit.
+ 
+ */
+import Foundation
+
+// NOTE: This implementation does not handle negative numbers
+public func radixSort(_ array: inout [Int] ) {
+  let radix = 10  //Here we define our radix to be 10
+  var done = false
+  var index: Int
+  var digit = 1  //Which digit are we on?
+  
+  
+  while !done {  //While our  sorting is not completed
+    done = true  //Assume it is done for now
+    
+    var buckets: [[Int]] = []  //Our sorting subroutine is bucket sort, so let us predefine our buckets
+    
+    for _ in 1...radix {
+      buckets.append([])
+    }
+    
+    
+    for number in array {
+      index = number / digit  //Which bucket will we access?
+      buckets[index % radix].append(number)
+      if done && index > 0 {  //If we arent done, continue to finish, otherwise we are done
+        done = false
+      }
+    }
+    
+    var i = 0
+    
+    for j in 0..<radix {
+      let bucket = buckets[j]
+      for number in bucket {
+        array[i] = number
+        i += 1
+      }
+    }
+    
+    digit *= radix  //Move to the next digit
+  }
+}

Radix Sort/RadixSort.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='macos'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>