top-k-frequent-elements solution

Author: ohgyulim

top-k-frequent-elements/ohgyulim.java

@@ -0,0 +1,36 @@
+import java.util.*;
+
+
+class Solution {
+    /* 시간 복잡도: O(N + M * log k)
+    * - for 루프: O(N), frequency 구하기
+    * - for 루프: O(M * log k), Map 순회
+        - Map 요소: M개
+    *   - PriorityQueue 연산 (offer, poll): 평균 O(logk)
+    * 
+    * 공간 복잡도: O(N + k), HashMap에 n개 + PriorityQueue에 k개
+    */ 
+    public int[] topKFrequent(int[] nums, int k) {
+        Map<Integer, Integer> frequencyMap = new HashMap<>();
+        // [num, frequency]
+        PriorityQueue<int[]> minHeap = new PriorityQueue<>((o1, o2) -> {
+            return o1[1] - o2[1];
+        });
+        for (int num : nums) {
+            frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1);
+        }
+        for (var entry: frequencyMap.entrySet()) {
+            minHeap.add(new int[]{entry.getKey(), entry.getValue()});
+            if (minHeap.size() > k) {
+                minHeap.poll();
+            }
+        }
+
+        int[] answer = new int[k];
+        for (int i = 0; i < k; i++) {
+            answer[i] = minHeap.poll()[0];
+        }
+        return answer;
+    }
+}
+