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);
       }
     }

test/algorithm_test.cc

@@ -114,12 +114,15 @@ TEST_SUITE("Algorithm") {
       {2, {3, 6}},
       {3, {4}},
       {4, {5}},
+      {5, {}},
       {6, {7, 9}},
       {7, {8}},
+      {8, {}},
       {9, {10}},
       {10, {11}},
       {11, {12}},
-      {12, {13}}
+      {12, {13}},
+      {13, {}}
     };
 
     CHECK_EQ(tree, expected);