python对dict进行排序：
result = sorted(my_dict.items(), key = lambda x:x[1], reverse=True)
如果对key排序,用x[0];默认的是从小到大排序,如果是从大到小,需要用reverse = True.
返回的result是一个list，不再是一个dict
words = ["i", "love", "leetcode", "i", "love", "coding"]
result = sorted(words.items(), key = lambda x:x[1], reverse=True)
result
返回的是
[('i', 2), ('love', 2), ('coding', 1), ('leetcode', 1)]