sort-characters-by-frequency.py

# Given a string s, sort it in decreasing order based on the frequency of the characters. The frequency of a character is the number of times it appears in the string.
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# Return the sorted string. If there are multiple answers, return any of them.
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# Example 1:
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# Input: s = "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.
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# Example 2:
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# Input: s = "cccaaa"
# Output: "aaaccc"
# Explanation: Both 'c' and 'a' appear three times, so both "cccaaa" and "aaaccc" are valid answers.
# Note that "cacaca" is incorrect, as the same characters must be together.
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# Example 3:
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# Input: s = "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.
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# Constraints:
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# 	1 <= s.length <= 5 * 105
# 	s consists of uppercase and lowercase English letters and digits.
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class Solution:
    def frequencySort(self, s: str) -> str:
        counter = collections.Counter(s)
        return ''.join([char*times for char, times in counter.most_common()])