python 根据两个字段排序, 一个升序, 一个降序

hackerrank 中有这样一个题:

给定一个字符串, 输出出现次数最多的前三个字符, 若两字符出现次数相同, 则按字典顺序排列.

# 样例输入

aabbbccde

# 样例输出

b 3
a 2
c 2

就是先将第二字段降序排序, 再将第一字段升序排序, 关键就是sorted函数key的指定, 可以用 lambda 或  operator.itemgetter

开始我是这样做的:

from collections import Counter
c = Counter(input())
l=sorted(c.items(), key=lambda s:(-s[1], s[0]))
for i in l[:3]:
    print(' '.join(map(str, list(i))))

EDITORIAL给出了三个参考解:

1.利用字母表

S = raw_input()
letters = [0]*26

for letter in S:
    letters[ord(letter)-ord('a')] += 1

for _ in range(3):

    max_letter = max(letters)

    for index in range(26):
        if max_letter == letters[index]:
            print chr(ord('a')+index), max_letter
            letters[index] = -1
            break

2.

from collections import Counter
from operator import itemgetter

for item in (sorted(sorted(Counter(raw_input()).items()), key = itemgetter(1), reverse = True)[:3]):
    print item[0], item[1]

3.

from collections import Counter

for letter, counts in sorted(Counter(raw_input()).most_common(),key = lambda x:(-x[1],x[0]))[:3]:
    print letter, counts

most_common 的行为可能在python 2和3之间不同, 没仔细研究

参考:

http://blog.chinaunix.net/xmlrpc.php?r=blog/article&uid=429659&id=3140368

http://stackoverflow.com/questions/6666748/python-sort-list-of-lists-ascending-and-then-decending

http://stackoverflow.com/questions/5212870/sorting-a-python-list-by-two-criteria