Fix topological sort bug, improve explanation

When concatenating the results from the individual depth-first searches, the subsequent results need to be prepended, not appended, to the final list.

Author: hollance

README.markdown

@@ -75,7 +75,7 @@ Special-purpose sorts:
 - Bucket Sort
 - Counting Sort
 - Radix Sort
-- [Topological Sort](Topological Sort/) :construction:
+- [Topological Sort](Topological Sort/)
 
 Bad sorting algorithms (don't use these!):
 

Topological Sort/Graph.swift

@@ -30,3 +30,22 @@ public class Graph: CustomStringConvertible {
     return nil
   }
 }
+
+extension Graph {
+  typealias InDegree = Int
+
+  func calculateInDegreeOfNodes() -> [Node : InDegree] {
+    var inDegrees = [Node : InDegree]()
+    
+    for (node, _) in adjacencyLists {
+      inDegrees[node] = 0
+    }
+    
+    for (_, adjacencyList) in adjacencyLists {
+      for nodeInList in adjacencyList {
+        inDegrees[nodeInList] = (inDegrees[nodeInList] ?? 0) + 1
+      }
+    }
+    return inDegrees
+  }
+}