leetcode 451 Sort Characters By Frequency
z
 2019-06-08 18:35:38  

Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

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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:

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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:

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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.
  • 非常自然的就想到了用Collections中的Counter来计数
  • Counter中有一个most_common()函数按照出现的次数进行排序
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class Solution(object):
    def frequencySort(self, s):
        """
        :type s: str
        :rtype: str
        """
        counter = collections.Counter(s)
        ans = ''
        for (c, n) in counter.most_common():
            ans += c * n
        return ans