Add single vertex contains method and replace vertices field with adjMatrix' keys

algorithm/tarjan.h

@@ -66,9 +66,9 @@ void Tarjan<T>::strongConnect(const T& v) {
 
 template<typename T>
 std::vector<SCC<T>> Tarjan<T>::execute() {
-  for (const auto& v : G.vertices) {
-    if (p[v].index == -1) {
-      strongConnect(v);
+  for (auto& v : G.adjMatrix) {
+    if (p[v.first].index == -1) {
+      strongConnect(v.first);
     }
   }