Added comments to python implementation of BFS

Author: hbostan

BFS/bfs.py

@@ -3,8 +3,11 @@
 
 def bfs(graph, root): 
     visited, queue = set(), collections.deque([root])
+    # While we still have nodes to visit
     while queue: 
         vertex = queue.popleft()
+        # Add neighbours of current node to queue if they
+        # haven't been visited yet and mark them as visited.
         for neighbour in graph[vertex]: 
             if neighbour not in visited: 
                 visited.add(neighbour)