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Python Solution:

class WordCount:
    def mapper(self, key, line):
        # key我們可以當(dāng)做是index，或者URL
        for word in line.split():
            yield word, key

    # @param key is from mapper
    # @param values is a set of value with the same key
    def reducer(self, key, values):
        # values 是一組index 或者 URL
        indexes = []
        for v in values:
            indexes.append(v)
        yield key, indexes