graph = { 'A':set(['B', 'C']),
         'B': set(['A', 'D', 'E']),
         'C': set(['A', 'F']),
         'D': set(['B']),
         'E': set(['B', 'F']),
         'F': set(['C', 'E'])}

s= 'A'
visited = set()
stack = [s]

while stack:
    vertex = stack.pop()
    if vertex not in visited:
        visited.add(vertex)
        stack.extend(graph[vertex] - visited)

print visited