Majority Frequency Characters — Python

# Time:  O(n + 26)
# Space: O(26)

import collections


# freq table
class Solution(object):
    def majorityFrequencyGroup(self, s):
        """
        :type s: str
        :rtype: str
        """
        cnt = collections.defaultdict(int)
        for x in s:
            cnt[x] += 1
        cnt2 = collections.defaultdict(list)
        for x, c in cnt.iteritems():
            cnt2[c].append(x)
        k = max(cnt2.iterkeys(), key=lambda x: (len(cnt2[x]), x))
        return "".join(cnt2[k])