不熟知识点复习

1.Python有序字典

dict = collections.OrderedDict()

dict.move_to_end(key)

dict.popitem(last=False) # last = False,先进先出.

2.

from collections import defaultdict
        graph = defaultdict(list)
        for x,y in edges:
            graph[x].append(y)
            graph[y].append(x)

heapq.nlargest：