critique?

longest-run.playground/Contents.swift

@@ -7,14 +7,37 @@ This excercise lends itself to TDD and we have provided a few basic tests below.
 import Cocoa
 
 func longestRun(whole: String) -> String {
-    return whole
+    let sorted = whole.characters.sort()
+
+    var currentRunCharacter = sorted.first
+    var currentRun = 0
+    var longestRunCharacter = currentRunCharacter
+    var longestRun = 0
+
+    if currentRunCharacter == nil {
+        return ""
+    }
+
+    sorted.forEach({(let char: Character) -> () in
+        if char == currentRunCharacter! {
+            currentRun += 1
+        } else {
+            currentRunCharacter = char
+            currentRun = 1
+        }
+        if longestRun < currentRun {
+            longestRun = currentRun
+            longestRunCharacter = currentRunCharacter
+        }
+    })
+    return String([Character](count: longestRun, repeatedValue: longestRunCharacter!))
 }
 
 // identity test
 var zzzz = "zzzz"
 assert(zzzz == longestRun(zzzz), "\(zzzz) should be the longest run of chars")
 
 // book
-// var book = "book"
-// assert("oo" == longestRun(book), "longest run in \(book) is 'oo'")
+var book = "book"
+assert("oo" == longestRun(book), "longest run in \(book) is 'oo'")