graph = {'0': set(['1', '2']),
         '1': set(['2']),
         '2': set(['0', '3']),
         '3': set(['3'])}
visited = {}

for each in graph:
    visited [each]=False

queue = []
s = '2'
queue.append(s)

while len(queue):
    visited[s]=True
    s= queue.pop(0)
    print s

    for each in graph[s]:
        if not visited[each]:
            visited[each]=True
            queue.append(each)