Minimum Rounds to Complete All Tasks — Python

# Time:  O(n)
# Space: O(n)

import collections


# math, freq table
class Solution(object):
    def minimumRounds(self, tasks):
        """
        :type tasks: List[int]
        :rtype: int
        """
        cnt = collections.Counter(tasks)
        return sum((x+2)//3 for x in cnt.itervalues()) if 1 not in cnt.itervalues() else -1