# Leetcode: Sort Characters By Frequency

Sort Characters By Frequency

Similar Problems:

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.
```

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

```## Blog link: https://code.dennyzhang.com/sort-characters-by-frequency
## Baisc Ideas: Sort with customized function
##
## Complexity:
import collections
class Solution(object):
def frequencySort(self, s):
"""
:type s: str
:rtype: str
"""
m = collections.defaultdict(lambda: 0)
for ch in s: m[ch] += 1
def myCompare(v1, v2):
if m[v1] > m[v2]: return -1
elif m[v1] < m[v2]: return 1
else:
if v1 == v2: return 0
return -1 if v1 < v2 else 1
return ''.join(sorted(s, cmp=myCompare))

# s = Solution()
# print s.frequencySort("Aabb") # "bbAa"
```

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