def sortedKeys(self):
        sortedItems = self.items()
        compare = lambda x, y: sign(y[1] - x[1])
        sortedItems.sort(cmp=compare)
        return [x[0] for x in sortedItems]
Returns a list of keys sorted by their values.  Keys
dataset/ETHPy150Open rlpy/rlpy/rlpy/Domains/PacmanPackage/util.py/Counter.sortedKeys