set1 = {1, 2, 3, (4, 5, 6), "good news"}

set2 = set([1, 2, 3])                 #相當于set2 = {1, 2, 3}， set函數(shù)只能傳入一個參數(shù)
set3 = set((1,2,3))　　                #相當于set3 = {1,2,3}
set4 = set({'a':1,'b':2,'c':3})　      #相當于set4 = {'a','b','c'}

>>> s = set([1, 1, 2, 2, 3, 3])
>>> s
{1, 2, 3}

>>> s = {1, 2, 3}
>>> s.add(4)
>>> s
{1, 2, 3, 4}
>>> s.add(4)
>>> s
{1, 2, 3, 4}

>>> s = {1, 2, 3, 4}
>>> s.remove(4)
>>> s
{1, 2, 3}

>>> s1 = set([1, 2, 3])
>>> s2 = set([2, 3, 4])
>>> s1 & s2
{2, 3}
>>> s1 | s2
{1, 2, 3, 4}

a=[1,6,7,4,4,5,4,5,4,5,5,6,7,8,5,6,7,3,4,2,2,1,4,8,9,4,5,6]

def get_datas(a):
    result = []
    data_dict = {}
    #鍵值對：鍵——數(shù)字，值——在列表中的次數(shù)
    #set(a)將列表轉化為set類型，并過濾掉其中重復的數(shù)字
    for item in set(a):
        data_dict[item] = a.count(item)

    #將鍵值對按值(數(shù)字出現(xiàn)的次數(shù))排序——從高到低排序
    #sorted為臨時性排序，不會改變原列表data_dict的順序
    res = sorted(data_dict.values(), reverse=True)
    for num in res:
        for key, value in data_dict.items():
            #key not in result能保證相同的數(shù)字只添加一次
            if num == value and key not in result:
                result.append(key)
    return result

result = get_datas(a)
print(result)