solve : DaleStudy#237 (Top K Frequent Elements) with Java

Author: kdh-92

top-k-frequent-elements/kdh-92.java

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+/**
+ * Constraints
+ * 1 <= nums.length <= 10^5
+ * -10^4 <= nums[i] <= 10^4
+ * k is in the range [1, the number of unique elements in the array].
+ * It is guaranteed that the answer is unique.
+ *
+ * Output
+ * - int 배열
+ */
+
+class Solution {
+    public int[] topKFrequent(int[] nums, int k) {
+        // (1) HashMap + PriorityQueue
+        // 시간복잡도 : O(N log N)
+        // Runtime : 15ms Beats 38.03%
+        // Memory : 48.93MB Beats 20.01%
+        Map<Integer, Integer> frequencyMap = new HashMap<>();
+
+        for (int num : nums) {
+            frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1);
+        }
+
+        PriorityQueue<Map.Entry<Integer, Integer>> minHeap = new PriorityQueue<>(Comparator.comparingInt(Map.Entry::getValue));
+
+        for (Map.Entry<Integer, Integer> entry : frequencyMap.entrySet()) {
+            minHeap.offer(entry);
+            if (minHeap.size() > k) {
+                minHeap.poll();
+            }
+        }
+
+        int[] result = new int[k];
+
+        for (int i = 0; i < k; i++) {
+            result[i] = minHeap.poll().getKey();
+        }
+
+        return result;
+
+        // (2) Stream
+        // 시간복잡도 : O(N log N)
+        // Runtime : 19ms Beats 8.16%
+        // Memory : 49.00MB Beats 20.01%
+        // Stream에 익숙해지기 위해 공부용
+         return Arrays.stream(nums)
+             .boxed()
+             .collect(Collectors.groupingBy(num -> num, Collectors.summingInt(num -> 1)))
+             .entrySet().stream()
+             .sorted((a, b) -> b.getValue() - a.getValue())
+             .limit(k)
+             .mapToInt(Map.Entry::getKey)
+             .toArray();
+
+        // (3) Array List
+        // 시간복잡도 : O(N)
+        // Runtime : 13ms Beats 75.77%
+        // Memory : 48.44MB Beats 61.68%
+         Map<Integer, Integer> frequencyMap = new HashMap<>();
+         for (int num : nums) {
+             frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1);
+         }
+
+         List<Integer>[] buckets = new List[nums.length + 1];
+         for (int key : frequencyMap.keySet()) {
+             int freq = frequencyMap.get(key);
+             if (buckets[freq] == null) {
+                 buckets[freq] = new ArrayList<>();
+             }
+             buckets[freq].add(key);
+         }
+
+         List<Integer> result = new ArrayList<>();
+         for (int i = buckets.length - 1; i >= 0 && result.size() < k; i--) {
+             if (buckets[i] != null) {
+                 result.addAll(buckets[i]);
+             }
+         }
+
+         return result.stream().mapToInt(Integer::intValue).toArray();
+    }
+}