Fix issues with std::range::views using clang and C++20

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() {