solve top-k-frequent elements

Author: samcho0608

top-k-frequent-elements/samcho0608.java

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+import java.util.*;
+
+class Solution {
+    // return: top k most freq elements
+    public int[] topKFrequent(int[] nums, int k) {
+        HashMap<Integer, Integer> freq = new HashMap<>();
+        // O (N)
+        for(int num : nums) {
+            freq.put(num, freq.getOrDefault(num, 0) + 1); // O(1)
+        }
+
+        // O (N log k)
+        PriorityQueue<int[]> heap = new PriorityQueue<>(Comparator.comparingInt(a -> a[1]));
+        for(int num : freq.keySet()) {
+            int f = freq.get(num); // O(1)
+            heap.add(new int[]{num, f}); // O(log k)
+
+            if(heap.size() > k) heap.poll(); // O(log k)
+        }
+
+        // O (N log k)
+        int[] result = new int[k];
+        for(int i = 0; i < k; i++) {
+            result[i] = heap.poll()[0]; // O(log k)
+        }
+
+        return result;
+    }
+}