对给出的列表进行词频统计，将单词与单词出现次数对应起来，以字典的形式输出
列表如下
l1=['a','a','b','b','b','c','c','c','c','d','d','d','d']
l1=['a','a','b','b','b','c','c','c','c','d','d','d','d'] print(l1) b=dict.fromkeys(l1,0) print(b) for x in l1: b[x]+=1 print(b)
输出结果：
['a', 'a', 'b', 'b', 'b', 'c', 'c', 'c', 'c', 'd', 'd', 'd', 'd'] {'a': 0, 'b': 0, 'c': 0, 'd': 0} {'a': 2, 'b': 3, 'c': 4, 'd': 4}
