Merge pull request #367 from bubbleship/main

Update 347-Top-K-Frequent-Elements.java

Author: neetcode-gh

java/347-Top-K-Frequent-Elements.java

@@ -1,4 +1,8 @@
-class Solution {
+class Solution1 {
+    /**
+     * Time Complexity: O(nlog(k))
+     * Space Complexity: O(n)
+     */
     public int[] topKFrequent(int[] nums, int k) {
         int[] arr = new int[k];
         HashMap<Integer, Integer> map = new HashMap<>();
@@ -17,3 +21,57 @@ public int[] topKFrequent(int[] nums, int k) {
         return arr;
     }
 }
+
+class Solution2 {
+    /**
+     * Time Complexity: O(n)
+     * Space Complexity: O(n)
+     */
+    public int[] topKFrequent(int[] numbers, int k) {
+		HashMap<Integer, Integer> map = new HashMap<>();
+		for (int number : numbers)
+			map.put(number, map.getOrDefault(number, 0) + 1);
+
+		int size = map.size();
+		int[] keys = new int[size];
+		int i = 0;
+		for (int key : map.keySet())
+			keys[i++] = key;
+
+		select(keys, map, 0, size - 1, size - k);
+		return Arrays.copyOfRange(keys, size - k, size);
+	}
+
+    // Modified implementation of Hoare's selection algorithm:
+    
+	private void select(int[] keys, Map<Integer, Integer> map, int left, int right, int kSmallest) {
+		while (left != right) {
+			int pivot = partition(keys, map, left, right, (left + right) / 2);
+
+			if (kSmallest == pivot) return;
+
+			if (kSmallest < pivot) right = pivot - 1;
+			else left = pivot + 1;
+		}
+	}
+
+	private int partition(int[] keys, Map<Integer, Integer> map,int left, int right, int pivot) {
+		int pivotValue = map.get(keys[pivot]);
+		swap(keys, pivot, right);
+		int index = left;
+
+		for (int i = left; i <= right; i++)
+			if (map.get(keys[i]) < pivotValue) {
+				swap(keys, i, index);
+				index++;
+			}
+		swap(keys, right, index);
+		return index;
+	}
+    
+    private void swap(int[] array, int i1, int i2) {
+		int temp = array[i1];
+		array[i1] = array[i2];
+		array[i2] = temp;
+	}
+}