Add vertices without arcs into the BFTree

algorithm/breadth_first_search.h

@@ -46,6 +46,7 @@ std::map<T, std::set<T>> BreadthFirstSearch<T>::execute(const T& root) {
       if (!visited[u]) {
         visited[u] = true;
         tree[v].insert(u);
+        tree[u];
         Q.push(u);
       }
     }