add solution: top-k-frequent-elements

Author: dusunax

top-k-frequent-elements/dusunax.py

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+'''
+# Leetcode 347. Top K Frequent Elements
+
+use **Counter** to count the frequency of each element in the list 🚀
+use **sorted()** to sort the elements by their frequency in descending order.
+
+## Time and Space Complexity
+
+```
+TC: O(n log n)
+SC: O(n)
+```
+
+### TC is O(n log n):
+- iterating through the list just once to count the frequency of each element. = O(n)
+- sorting the elements by their frequency in descending order. = O(n log n)
+
+### SC is O(n):
+- using a Counter to store the frequency of each element. = O(n)
+- sorted() creates a new list that holds the elements of frequency_map. = O(n)
+- result list that holds the top k frequent elements. = O(k)
+'''
+
+class Solution:
+    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
+        frequency_map = Counter(nums) # TC: O(n), SC: O(n)
+        sorted_frequencies = sorted(frequency_map.items(), key=lambda x: x[1], reverse=True) # TC: O(n log n), SC: O(n)
+        
+        result = [] # SC: O(k)
+        for e in sorted_frequencies: # TC: O(k)
+            result.append(e[0])
+
+        return result[0:k]