[Swift 4] Update Topological Sort

Author: remlostime

Topological Sort/Topological Sort.playground/Contents.swift

@@ -1,5 +1,10 @@
 //: Playground - noun: a place where people can play
 
+// last checked with Xcode 9.0b4
+#if swift(>=4.0)
+print("Hello, Swift 4!")
+#endif
+
 import UIKit
 
 let graph = Graph()