update to radix sort

tracking the largest value in the array on the first iteration of the bucket sort allows you to do one less loop and drops the need for a `done` variable.
also updated the bucket initialization to use the `(repeatingElement: count:)` initializer and switched to using an iterator for looping through the buckets instead of accessing them based on the range loop.

Author: jakecast

Radix Sort/radixSort.swift

@@ -5,43 +5,35 @@
 */
 
 
-func radixSort(inout arr: [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 arr {
-      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 {
-        arr[i] = number
-        i += 1
-      }
-    }
-
-    digit *= radix  //Move to the next digit
-  }
+func radixSort(arr: inout [Int]) {
+    let radix = 10
+
+    var digit: Int = 1
+    var largest: Int = 0
+
+    var buckets: [[Int]]
+    var index: Int
+    var pos: Int
+
+    repeat {
+        pos = 0
+        buckets = Array(repeatElement([], count: 10))
+
+        for num in arr {
+            index = num / digit
+            buckets[index % radix].append(num)
+
+            if digit == 1 {
+                largest = max(largest, num)
+            }
+        }
+        for bin in buckets {
+            for num in bin {
+                arr[pos] = num
+                pos = pos + 1
+            }
+        }
+
+        digit = digit * radix
+    } while digit < largest
 }