Solution for Top K Frequent Elements #237

Author: dohyeon2

top-k-frequent-elements/dohyeon2.java

@@ -0,0 +1,42 @@
+import java.util.HashMap;
+import java.util.Map;
+import java.util.PriorityQueue;
+import java.util.Comparator;
+
+public class dohyeon2 {
+    public int[] topKFrequent(int[] nums, int k) {
+        // approach : 
+        // 1. create map to match num to frequency
+        // 2. create max frequency Priority Queue for sort
+        // 3. pop the max value from the queue until k
+        // time complexity O(n log n) : priority queue comparison
+        // space complexity O(n)
+
+        // ChatGPT says that there is O(n) solution for this, how?
+        HashMap<Integer, Integer> frequentMap = new HashMap<>();
+
+        for (int i = 0; i < nums.length; i++) {
+            int num = nums[i];
+            frequentMap.merge(num, 1, Integer::sum);
+        }
+
+        PriorityQueue<Map.Entry<Integer, Integer>> PQ = new PriorityQueue<>(k,
+                Comparator.comparing(Map.Entry<Integer, Integer>::getValue).reversed());
+
+        for(Map.Entry<Integer,Integer> e : frequentMap.entrySet()){
+            PQ.add(e);
+        }
+
+        int[] result = new int[k];
+
+        int idx = 0;
+
+        while (idx < k && PQ.size() > 0) {
+            Map.Entry<Integer, Integer> v = PQ.poll();
+            result[idx] = (int) v.getKey();
+            idx++;
+        }
+
+        return result;
+    }
+}