Refactor: Replace std::map and std::set with unordered versions

graph/breadth_first_tree.h

@@ -10,7 +10,7 @@ class BreadthFirstTree : public Digraph<T> {
 public:
   BreadthFirstTree() = default;
 
-  BreadthFirstTree(std::map<T, std::set<T>> G, T root)
+  BreadthFirstTree(std::unordered_map<T, std::unordered_set<T>> G, T root)
     : BreadthFirstTree<T>(Digraph<T>(G), root) {}
 
   BreadthFirstTree(Digraph<T> G, T root);