每日一题

 1 class Solution:
 2     def sortString(self, s):
 3         num = [0] * 26
 4         for ch in s:
 5             num[ord(ch) - ord(‘a‘)] += 1
 6         
 7         ret = list()
 8         while len(ret) < len(s):
 9             for i in range(26):
10                 if num[i]:
11                     ret.append(chr(i + ord(‘a‘)))
12                     num[i] -= 1
13             for i in range(25, -1, -1):
14                 if num[i]:
15                     ret.append(chr(i + ord(‘a‘)))
16                     num[i] -= 1
17 
18         return "".join(ret)