LeetCode 451. Sort Characters By Frequency

问题描述：

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.

思路：这道题思路还是比较普通的，没什么tricks，首先用一个dict统计字符出现的频率，然后把dict根据value值排序得到r，r的格式就像这样['e':2,'t':1,'r':1]，然后按顺序拼拼接str返回即可

class Solution(object):

    def frequencySort(self, s):
        d = {}
        for c in s:
            if c in d:
                d[c] += 1
            else:
                d[c] = 1
        print(d)
        r = sorted(d.items(),key=lambda item:item[1],reverse=True)
        s=''
        for i in r:
            s+=(i[0]*i[1])
        return s