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.

class Solution(object):
    def frequencySort(self, s):
        d = {}
        
        for i in s:
            d[i] = d.get(i,0) + 1
            
        l = sorted(d.iteritems(),key = lambda k:k[1],reverse = True)
        
        res = ''
        for (k,v) in l:
            res += k * v
        return res
        
        """
        :type s: str
        :rtype: str
        """