Sort Characters By Frequency

Author: anantkaushik

451-sort-characters-by-frequency.py

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+"""
+Problem Link: https://leetcode.com/problems/sort-characters-by-frequency/
+
+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.
+"""
+class Solution:
+    def frequencySort(self, s: str) -> str:
+        d1, d2 = {}, {}
+        for c in s:
+          d1[c] = d1.get(c, 0) + 1
+        
+        for k,v in d1.items():
+          d2[v] = d2.get(v, '') + k*v
+        
+        res = ''
+        for i in range(len(s), -1, -1):
+          if i in d2:
+            res += d2[i]
+        return res