diff --git a/chapter6/ex6.1.py b/chapter6/ex6.1.py
index 52e9b53..88b0e06 100644
--- a/chapter6/ex6.1.py
+++ b/chapter6/ex6.1.py
@@ -21,7 +21,7 @@ graph = {
     "f": [],
 }
 
-visited = []
+visited = set()
 
 
 def bfs():
@@ -36,7 +36,7 @@ def bfs():
 
         if current_node.name in visited:
             continue
-        visited.append(current_node.name)
+        visited.add(current_node.name)
 
         next_nodes = graph[current_node.name]
         for node in next_nodes:
diff --git a/chapter6/ex6.2.py b/chapter6/ex6.2.py
index cebf63d..c2f74b7 100644
--- a/chapter6/ex6.2.py
+++ b/chapter6/ex6.2.py
@@ -21,7 +21,7 @@ graph = {
     "bat": [],
 }
 
-visited = []
+visited = set()
 
 
 def bfs():
@@ -36,7 +36,7 @@ def bfs():
 
         if current_node.name in visited:
             continue
-        visited.append(current_node.name)
+        visited.add(current_node.name)
 
         next_nodes = graph[current_node.name]
         for node in next_nodes: