sort-characters-by-frequency.py

# Time:  O(n)
# Space: O(n)

# 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):
        """
        :type s: str
        :rtype: str
        """
        freq = collections.defaultdict(int)
        for c in s:
            freq[c] += 1
            
        counts = [""] * (len(s)+1)
        for c in freq:
            counts[freq[c]] += c
            
        result = ""
        for count in reversed(xrange(len(counts)-1)):
            for c in counts[count]:
                result += c * count
        
        return result