热门标签追踪 (Top-K)

import unittest
from typing import List

def get_trending_hashtags(stream: List[str], k: int) -> List[str]:
    """
    Finds the k most frequent hashtags in a stream.
    
    Args:
        stream: A list of strings representing hashtags.
        k: The number of top hashtags to return.
        
    Returns:
        A list of the k most frequent hashtags in descending order of frequency.
        Ties are broken lexicographically.
    """
    # Your code here
    pass


class TestTrendingHashtags(unittest.TestCase):
    def test_basic(self):
        stream = ["#emergence", "#science", "#ai", "#science", "#emergence", "#science"]
        k = 2
        expected = ["#science", "#emergence"]
        self.assertEqual(get_trending_hashtags(stream, k), expected)

    def test_tie_breaking(self):
        stream = ["#a", "#b", "#c", "#a", "#b"]
        k = 2
        # Frequencies: #a: 2, #b: 2, #c: 1
        # Top 2 with tie-breaking (lexicographical): ["#a", "#b"]
        expected = ["#a", "#b"]
        self.assertEqual(get_trending_hashtags(stream, k), expected)

    def test_single_element(self):
        stream = ["#test"]
        k = 1
        expected = ["#test"]
        self.assertEqual(get_trending_hashtags(stream, k), expected)
        
    def test_large_k(self):
        stream = ["#apple", "#banana", "#cherry"]
        k = 5
        # k is larger than unique elements, return all unique in order
        expected = ["#apple", "#banana", "#cherry"]
        self.assertEqual(get_trending_hashtags(stream, k), expected)

if __name__ == '__main__':
    unittest.main()