一个 Counter 是一个 dict 的子类，用于计数可哈希对象.
    统计元素出现次数：Counter

    1. from collections import Counter
    2. new_list = ['a', 'a', 'a', 'b', 'b', 'b', 'c']
    3. print(Counter(new_list))    # 返回一个Counter对象
    5. """
    6. Counter({'a': 3, 'b': 3, 'c': 1})
    7. """

    展开每个计数值大于等于1的元素：elements()

    1. new_counter = Counter(new_list)
    2. print(new_counter.elements())   # 返回一个迭代器
    3. """<itertools.chain object at 0x7ffde8262050>"""
    5. print(list(new_counter.elements()))
    6. """['a', 'a', 'a', 'b', 'b', 'b', 'c']"""

    统计元素出现最多的元素：Counter.most_common()
    返回一个列表，其中包含 n 个最常见的元素及出现次数，按常见程度由高到低排序

    1. from collections import Counter
    2. list_info = 'hello worldeeeee'
    3. list1 = [1, 3, 3, 4, 1, 4]
    4. print(Counter(list_info))
    5. print(Counter(list_info).most_common()[0][0])
    6. print(max(set(list_info), key=list_info.count))
    7. -------------------------
    8. #Counter({'e': 6, 'l': 3, 'o': 2, 'h': 1, ' ': 1, 'w': 1, 'r': 1, 'd': 1})
    9. #e
    10. #e