擅长:python、mysql、java
<pre><code>>>> import collections
>>> out = collections.defaultdict(set)
>>> inval = {'Europa': 'Jupiter', 'Psamathe': 'Neptune', 'Cordelia': 'Uranus', 'Cupid': 'Uranus'}
>>> for k, v in inval.items():
... out[v].add(k)
...
>>> out
defaultdict(<type 'set'>, {'Jupiter': set(['Europa']), 'Neptune': set(['Psamathe']), 'Uranus': set(['Cordelia', 'Cupid'])})
</code></pre>