Approach #1 :(Dictionary)
Traverse string s and store each letter and their number of occurrence in a dictionary. Then sort the dictionary by values in descending order and print the letters.
"""
import operator
class Solution(object):
def frequencySort(self, s):
mydict = dict()
for i in xrange(len(s)):
mydict[s[i]] = mydict.get(s[i],0) + 1
sorted_list = sorted(mydict.items(), key=operator.itemgetter(1),reverse=True)
retval = ""
for w in sorted_list:
retval = retval + w[0]*w[1]
return retval
"""