题目

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.

难度

Medium

方法

对每个字符次数统计，排序后组成新的字符串即可。借用python的Counter和most_common()方法，非常简单

python代码

import collections

class Solution(object):
    def frequencySort(self, s):
        """
        :type s: str
        :rtype: str
        """

        return "".join([char*times for char, times in collections.Counter(s).most_common()])

assert Solution().frequencySort("ttreee") == "eeettr"