451.Sort Characters By Frequency(Medium)

Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:
"tree"

Output:
"eert"

Explanation:
'e' appears twice while 'r' and 't' both appear once. So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer. 

Example 2:

Input:
"cccaaa"

Output:
"cccaaa"

Explanation:
Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer. Note that "cacaca" is incorrect, as the same characters must be together. 

Example 3:

Input:
"Aabb"

Output:
"bbAa"

Explanation:
"bbaA" is also a valid answer, but "Aabb" is incorrect. Note that 'A' and 'a' are treated as two different characters.

题意：
对于一个给定的字符串，将字符按照所出现的次数降序排列

思路：
可以考虑先进行字符统计，然后再降序排列

import collections

class Solution:
    def frequencySort(self, s):
        return ''.join(c * t for c, t in collections.Counter(s).most_common())

提示：

在Python中，collections模块下的Counter类（计数器）是一个容器，用来跟踪值出现的次数，是字典dict的子类，用来为hashtable对象计数。