Most Common Word — Python

# Time:  O(m + n), m is the size of banned, n is the size of paragraph
# Space: O(m + n)

import collections


class Solution(object):
    def mostCommonWord(self, paragraph, banned):
        """
        :type paragraph: str
        :type banned: List[str]
        :rtype: str
        """
        lookup = set(banned)
        counts = collections.Counter(word.strip("!?',.")
                                     for word in paragraph.lower().split())

        result = ''
        for word in counts:
            if (not result or counts[word] > counts[result]) and \
               word not in lookup:
                result = word
        return result