"""pseudo-code"""

"""
BFS(graph G, start vertex s):
// all nodes initially unexplored
mark s as explored
let Q = queue data structure, initialized with s
while Q is non-empty:
 remove the first node of Q, call it v
 for each edge(v, w): // for w in graph[v]
 if w unexplored:
 mark w as explored
 add w to Q (at the end)

"""

import collections


def bfs(graph, start):
 explored, queue = set(), [start] # collections.deque([start])
 explored.add(start)
 while queue:
 v = queue.pop(0) # queue.popleft()
 for w in graph[v]:
 if w not in explored:
 explored.add(w)
 queue.append(w)
 return explored


G = {'A': ['B', 'C'],
 'B': ['A', 'D', 'E'],
 'C': ['A', 'F'],
 'D': ['B'],
 'E': ['B', 'F'],
 'F': ['C', 'E']}

print(bfs(G, 'A'))