Merge pull request #595 from remlostime/tree-swift

[Swift 4] Update Minimum Spanning Tree(unweighted)

Author: kelvinlauKL

Minimum Spanning Tree (Unweighted)/MinimumSpanningTree.playground/Pages/Minimum spanning tree example.xcplaygroundpage/Contents.swift

@@ -1,3 +1,8 @@
+// last checked with Xcode 9.0b4
+#if swift(>=4.0)
+print("Hello, Swift 4!")
+#endif
+
 func breadthFirstSearchMinimumSpanningTree(_ graph: Graph, source: Node) -> Graph {
   let minimumSpanningTree = graph.duplicate()