def mystere(self):
    from collections import deque
    sortie = []
    tmp = deque()
    tmp.append('NA')
    while tmp:
        S = tmp.popleft()
        if S not in sortie:
            sortie.append(S)
            unvisited = [n for n in self.sommets[S] if n not in sortie]
            tmp.extend(unvisited)
    return sortie