diff --git a/include/algorithm/tarjan.h b/include/algorithm/tarjan.h
index ff047c9..d86fe35 100644
--- a/include/algorithm/tarjan.h
+++ b/include/algorithm/tarjan.h
@@ -70,9 +70,9 @@ template <typename T> void Tarjan<T>::strongConnect(const T &u) {
 }
 
 template <typename T> auto Tarjan<T>::execute() {
-  for (const auto &u : std::views::keys(adjList)) {
-    if (V[u].index == -1)
-      strongConnect(u);
+  for (const auto &u : adjList) {
+    if (V[u.first].index == -1)
+      strongConnect(u.first);
   }
   return SCCs;
 }
diff --git a/include/graph/graph.h b/include/graph/graph.h
index fca956a..a29594e 100644
--- a/include/graph/graph.h
+++ b/include/graph/graph.h
@@ -3,6 +3,7 @@
 
 #include <ostream>
 #include <ranges>
+#include <vector>
 #include <unordered_map>
 #include <unordered_set>
 
@@ -38,8 +39,14 @@ public:
   std::unordered_map<T, std::unordered_set<T>> adjList;
 };
 
-template <typename T> auto Graph<T>::vertices() const {
-  return std::views::keys(adjList);
+template <typename T>
+auto Graph<T>::vertices() const {
+    std::vector<T> keys;
+    keys.reserve(adjList.size());
+    for (const auto& pair : adjList) {
+        keys.push_back(pair.first);
+    }
+    return keys;
 }
 
 template <typename T> std::uint16_t Graph<T>::V() {
diff --git a/test/algorithm_test.cc b/test/algorithm_test.cc
index 8f68d4b..8c30528 100644
--- a/test/algorithm_test.cc
+++ b/test/algorithm_test.cc
@@ -3,6 +3,7 @@
 #include "algorithm/tarjan.h"
 #include "algorithm/breadth_first_search.h"
 
+#include <memory>
 #include <random>
 #include <functional>
 #include <iostream>
@@ -51,7 +52,7 @@ TEST_SUITE("Algorithm") {
     };
 
     for (auto i = 0; i < SCCs.size(); i++) {
-      auto kv = std::views::keys(SCCs[i].adjList);
+      auto kv = SCCs[i].vertices();
       CHECK_EQ(std::is_permutation(kv.begin(), kv.end(),
                                    expected[i].begin()), true);
     }