refactor 215

Author: fishercoder1534

src/main/java/com/fishercoder/solutions/_215.java

@@ -1,5 +1,6 @@
 package com.fishercoder.solutions;
 
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.PriorityQueue;
 
@@ -17,15 +18,66 @@
  */
 public class _215 {
 
-    public int findKthLargest(int[] nums, int k) {
-        PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Collections.reverseOrder());
-        for (int i : nums) {
-            maxHeap.offer(i);
+    public static class Solution1 {
+        public int findKthLargest(int[] nums, int k) {
+            Arrays.sort(nums);
+            return nums[nums.length - k];
         }
-        while (k-- > 1) {
-            maxHeap.poll();
+    }
+
+    public static class Solution2 {
+        public int findKthLargest(int[] nums, int k) {
+            PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Collections.reverseOrder());
+            for (int i : nums) {
+                maxHeap.offer(i);
+            }
+            while (k-- > 1) {
+                maxHeap.poll();
+            }
+            return maxHeap.poll();
         }
-        return maxHeap.poll();
     }
 
+    public static class Solution3 {
+        /**Quick Select algorithm
+         * Time: O(n) in average, O(n^2) in worst case
+         * 
+         * Reference: https://discuss.leetcode.com/topic/14611/java-quick-select*/
+        public int findKthLargest(int[] nums, int k) {
+            int start = 0;
+            int end = nums.length - 1;
+            int index = nums.length - k;
+            while (start < end) {
+                int pivot = partition(nums, start, end);
+                if (pivot < index) start = pivot + 1;
+                else if (pivot > index) end = pivot - 1;
+                else return nums[pivot];
+            }
+            return nums[start];
+        }
+
+        int partition(int[] nums, int start, int end) {
+            int pivot = start;
+            while (start <= end) {
+                while (start <= end && nums[start] <= nums[pivot]) {
+                    start++;
+                }
+                while (start <= end && nums[end] > nums[pivot]) {
+                    end--;
+                }
+                if (start > end) {
+                    break;
+                }
+                swap(nums, start, end);
+            }
+            swap(nums, end, pivot);
+            return end;
+        }
+
+        void swap(int[] nums, int i, int j) {
+            int temp = nums[i];
+            nums[i] = nums[j];
+            nums[j] = temp;
+        }
+    }
 }

src/test/java/com/fishercoder/_215Test.java

@@ -10,22 +10,22 @@
  * Created by fishercoder on 5/9/17.
  */
 public class _215Test {
-    private static _215 test;
+    private static _215.Solution2 solution2;
     private static int k;
     private static int[] nums;
     private static int actual;
     private static int expected;
 
     @BeforeClass
     public static void setup(){
-        test = new _215();
+        solution2 = new _215.Solution2();
     }
 
     @Test
     public void test1(){
         k = 2;
         nums = new int[]{3,2,1,5,6,4};
-        actual = test.findKthLargest(nums, k);
+        actual = solution2.findKthLargest(nums, k);
         expected = 5;
         assertEquals(expected, actual);
     }