update solution: top-k-frequent-elements

Author: dusunax

top-k-frequent-elements/dusunax.py

@@ -2,23 +2,44 @@
 # 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.
+
+- **solution 1**: use **sorted()** to sort the elements by their frequency in descending order.
+- **solution 2**: use **bucket sort** to sort the elements by their frequency in descending order. (efficient!)
 
 ## Time and Space Complexity
 
+### solution 1: topKFrequent()
+
 ```
 TC: O(n log n)
 SC: O(n)
 ```
 
-### TC is O(n log 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):
+#### 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)
+
+### solution 2: topKFrequentBucketSort()
+
+```
+TC: O(n)
+SC: O(n)
+```
+
+#### TC is O(n):
+- iterating through the list just once to count the frequency of each element. = O(n)
+- creating **buckets** to store the elements by their frequency. = O(n)
+- iterating through the buckets in reverse order to get only the top k frequent elements. = O(n)
+
+#### SC is O(n):
+- using a Counter to store the frequency of each element. = O(n)
+- using buckets to store the elements by their frequency. = O(n)
+- result list that holds only the top k frequent elements. = O(k)
 '''
 
 class Solution:
@@ -31,3 +52,18 @@ def topKFrequent(self, nums: List[int], k: int) -> List[int]:
             result.append(e[0])
 
         return result[0:k]
+    
+    def topKFrequentBucketSort(self, nums: List[int], k: int) -> List[int]:
+        frequency_map = Counter(nums)
+        n = len(nums)
+        buckets = [[] for _ in range(n + 1)]
+
+        for num, freq in frequency_map.items():
+            buckets[freq].append(num)
+
+        result = []
+        for i in range(len(buckets) - 1, 0, -1):
+            for num in buckets[i]:
+                result.append(num)
+                if len(result) == k:
+                    return result