Add reverse a digraph function and test

graph/digraph.h

@@ -28,7 +28,15 @@ Digraph<T>::Digraph(std::map<T, std::set<T>> digraph) {
 
 template<typename T>
 Digraph<T> Digraph<T>::reverse() {
-  return Digraph<T>();
+  std::map<T, std::set<T>> revMatrix;
+  
+  for (const auto& v : Graph<T>::adjMatrix) {
+    for (const auto& u : v.second) {
+      revMatrix[u].insert(v.first);
+    }
+  }
+
+  return revMatrix;
 }
 
 } // namespace graph