Java solution 347

Author: gavinfish

java/_347TopKFrequentElements.java

@@ -0,0 +1,38 @@
+import java.util.*;
+
+/**
+ * Given a non-empty array of integers, return the k most frequent elements.
+ * <p>
+ * For example,
+ * Given [1,1,1,2,2,3] and k = 2, return [1,2].
+ * <p>
+ * Note:
+ * You may assume k is always valid, 1 ≤ k ≤ number of unique elements.
+ * Your algorithm's time complexity must be better than O(n log n), where n is the array's size.
+ * <p>
+ * Created by drfish on 15/06/2017.
+ */
+public class _347TopKFrequentElements {
+    public List<Integer> topKFrequent(int[] nums, int k) {
+        List<Integer> result = new ArrayList<>();
+        Map<Integer, Integer> map = new HashMap<>();
+        PriorityQueue<Map.Entry<Integer, Integer>> maxHeap = new PriorityQueue<>((n1, n2) -> n2.getValue() -
+                n1.getValue());
+        for (int num : nums) {
+            map.put(num, map.getOrDefault(num, 0) + 1);
+        }
+        for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
+            maxHeap.offer(entry);
+        }
+        while (k > 0) {
+            k--;
+            result.add(maxHeap.poll().getKey());
+        }
+        return result;
+    }
+
+    public static void main(String[] args) {
+        _347TopKFrequentElements solution = new _347TopKFrequentElements();
+        assert Arrays.asList(1, 2).equals(solution.topKFrequent(new int[]{1, 1, 1, 2, 2, 3}, 2));
+    }
+}