Fix infinite recursion in DFS

Fixes infinite recursion in the DFS algorithm for topological sort caused by graphs with cycles. 
For example the following graph would previously have recursed indefinitely `a->b->a`.

Author: litso

Diff for: Topological Sort/TopologicalSort1.swift

@@ -1,6 +1,7 @@
 extension Graph {
   private func depthFirstSearch(_ source: Node, visited: inout [Node : Bool]) -> [Node] {
     var result = [Node]()
+    visited[source] = true
 
     if let adjacencyList = adjacencyList(forNode: source) {
       for nodeInAdjacencyList in adjacencyList {
@@ -10,7 +11,6 @@ extension Graph {
       }
     }
 
-    visited[source] = true
     return [source] + result
   }