diff --git a/solutions/system_design/social_graph/social_graph_snippets.py b/solutions/system_design/social_graph/social_graph_snippets.py
index f1cc696..8a83f74 100644
--- a/solutions/system_design/social_graph/social_graph_snippets.py
+++ b/solutions/system_design/social_graph/social_graph_snippets.py
@@ -1,4 +1,6 @@
 # -*- coding: utf-8 -*-
+from collections import deque
+
 
 class Graph(object):