Add State(Enum) to social_graph_snippets.py

* Fixes #148
* Enables #93
* Replaces #162

solutions/system_design/social_graph/social_graph_snippets.py

@@ -1,5 +1,11 @@
 # -*- coding: utf-8 -*-
 from collections import deque
+from enum import Enum
+
+
+class State(Enum):
+    unvisited = 0
+    visited = 1
 
 
 class Graph(object):