[Swift 4] Update Red Black Tree

Author: remlostime

Red-Black Tree/RedBlackTree.playground/Contents.swift

@@ -2,6 +2,11 @@
 // Test for  the RedBlackTree implementation provided in the Source folder of this Playground
 import Foundation
 
+// last checked with Xcode 9.0b4
+#if swift(>=4.0)
+print("Hello, Swift 4!")
+#endif
+
 let redBlackTree = RedBlackTree<Double>()
 
 let randomMax = Double(0x10000000)
@@ -27,4 +32,4 @@ for i in 0..<1000 {
     redBlackTree.verify()
   }
 }
-redBlackTree.verify()
+redBlackTree.verify()