Top K frequent words

这个题早该出现了，直接用堆统计就行了

class Solution(object):
    def topKFrequent(self, words, k):
        """
        :type words: List[str]
        :type k: int
        :rtype: List[str]
        """
        store = collections.Counter()
        for word in words:
            store[word] += 1

        result = []
        heap = []
        for key, value in store.iteritems():
            heapq.heappush(heap, (-value, key), )

        while k:
            result.append(heapq.heappop(heap)[1])
            k -= 1

        return result