Top K frequenct elements

Author: JoshuaJiangFD

src/main/java/joshua/leetcode/design/MedianFinder.java

@@ -2,6 +2,8 @@
 
 package joshua.leetcode.design;
 
+import joshua.leetcode.solutiontag.MinHeap;
+
 import java.util.PriorityQueue;
 
 /**
@@ -53,10 +55,12 @@ public abstract class MedianFinder {
      * 如果两个堆大小相差1，则median就是aHeap的栈顶元素。
      *
      * 在Java中，PriorityQueue的内部实现就是min Heap。
+     *
      * 这样PriorityQueue的头部对应min heap的栈顶。压入栈的操作等同于入队操作，优先队列会调整栈顶元素使其最小。
      * 至于MaxHeap可以使用元素的负值插入到priorityQueue中，这样队列头部的元素的相反值就是最大值了，实现了max heap的作用。
      *
      */
+    @MinHeap
     public static class Solution1 extends MedianFinder {
 
         /**

src/main/java/joshua/leetcode/heap/TopKFrequentElements.java

@@ -0,0 +1,100 @@
+package joshua.leetcode.heap;
+
+import com.google.common.collect.Lists;
+import joshua.leetcode.solutiontag.BucketSort;
+import joshua.leetcode.solutiontag.MinHeap;
+
+import java.util.*;
+
+/**
+ * 347. Top K Frequent Elements</br>
+ * <p/>
+ * <a href = "https://leetcode.com/problems/top-k-frequent-elements/">leetcode link</a>
+ * <p/>
+ * Created by joshu on 2016/5/24.
+ */
+public abstract class TopKFrequentElements {
+
+    public abstract List<Integer> topKFrequent(int[] nums, int k);
+
+
+    @MinHeap
+    public static class Solution1 extends TopKFrequentElements {
+
+        @Override
+        public List<Integer> topKFrequent(int[] nums, int k) {
+            Arrays.sort(nums);
+            // in java priority queue is implemented by min stack, the element at head is the minimum of queue.
+            PriorityQueue<QueueItem> minStack = new PriorityQueue<QueueItem>();
+            for (int i = 0; i < nums.length; i++) {
+                int count = 1;
+                while (i + 1 < nums.length && nums[i] == nums[i + 1]) {
+                    count++;
+                    i++;
+                }
+                minStack.offer(new QueueItem(nums[i], count));
+            }
+            List<Integer> topK = Lists.newArrayList();
+            for (int j = 0; j < k; j++) {
+                topK.add(minStack.poll().val);
+            }
+            return topK;
+        }
+
+        class QueueItem implements Comparable<QueueItem> {
+            int val;
+            int frequency;
+
+            public QueueItem(int val, int frequency) {
+                this.val = val;
+                this.frequency = frequency;
+            }
+
+            @Override
+            public int compareTo(QueueItem o) {
+                return o.frequency - frequency;
+            }
+        }
+    }
+
+    @BucketSort
+    public static class Solution2 extends TopKFrequentElements {
+
+        @Override
+        public List<Integer> topKFrequent(int[] nums, int k) {
+            Arrays.sort(nums);
+            Map<Integer, List<Integer>> frequencyBucket = new HashMap<Integer, List<Integer>>();
+            int maxFrequency = 0;
+            for (int i = 0; i < nums.length; i++) {
+                int count = 1;
+                while (i + 1 < nums.length && nums[i] == nums[i + 1]) {
+                    count++;
+                    i++;
+                }
+                if (!frequencyBucket.containsKey(count)) {
+                    frequencyBucket.put(count, new ArrayList<Integer>());
+                }
+                frequencyBucket.get(count).add(nums[i]);
+                maxFrequency = Math.max(maxFrequency, count);
+            }
+            List<Integer> topK = new ArrayList<Integer>();
+            int size = 0;
+            for (int i = maxFrequency; i > 0; i--) {
+                if (!frequencyBucket.containsKey(i)) {
+                    continue;
+                }
+                for (Integer ele : frequencyBucket.get(i)) {
+                    topK.add(ele);
+                    size++;
+                    if (size == k) {
+                        break;
+                    }
+                }
+                if (size == k) {
+                    break;
+                }
+            }
+            return topK;
+        }
+    }
+}

src/main/java/joshua/leetcode/solutiontag/BucketSort.java

@@ -0,0 +1,7 @@
+package joshua.leetcode.solutiontag;
+
+/**
+ * Created by joshu on 2016/5/24.
+ */
+public @interface BucketSort {
+}

src/main/java/joshua/leetcode/solutiontag/MinHeap.java

@@ -0,0 +1,7 @@
+package joshua.leetcode.solutiontag;
+
+/**
+ * Created by joshu on 2016/5/24.
+ */
+public @interface MinHeap {
+}

src/test/java/joshua/leetcode/heap/TopKFrequentElementsTest.java

@@ -0,0 +1,28 @@
+package joshua.leetcode.heap;
+
+import com.google.common.collect.Lists;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.List;
+
+import static org.junit.Assert.*;
+
+/**
+ * Created by joshu on 2016/5/24.
+ */
+public class TopKFrequentElementsTest {
+
+    TopKFrequentElements solution;
+
+    @Before
+    public void setUp() {
+        solution = new TopKFrequentElements.Solution2();
+    }
+
+    @Test
+    public void testSolution() {
+        List<Integer> result = solution.topKFrequent(new int[]{1, 1, 1, 2, 2, 3}, 2);
+        assertEquals(result, Lists.newArrayList(1, 2));
+    }
+}