states_needed = set(["mt","wa","or","id","nv","ut","ca","az"])
stations = {}
stations["kone"] = set(["id", "nv", "ut"])
stations["ktwo"] = set(["wa", "id", "mt"])
stations["kthree"] = set(["or", "nv", "ca"])
stations["kfour"] = set(["nv", "ut"])
stations["kfive"] = set(["ca", "az"])
final_stations = set()
while states_needed:
best_station = None
states_covered = set()
for station, states in stations.items():
covered = states_needed & states
print covered
if len(covered) > len(states_covered):
best_station = station
states_covered = covered
states_needed -= states_covered
final_stations.add(best_station)
print states_needed
print final_stations
'''
先求最大交集，再减。再次同样的步骤，直到states_needed全集为空
'''
