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

sortie = []
file = deque()
file.append('A')

while file:
    S = file.popleft()
    if S not in sortie:
        sortie.append(S)
        unvisited = [n for n in G[S] if n not in sortie]
        file.extend(unvisited)

print(sortie)