fd

Author: dingjikerbo

leetcode/solution/src/KthLargestElementInArray.java

@@ -1,5 +1,6 @@
 import java.util.Arrays;
 import java.util.PriorityQueue;
+import java.util.Random;
 
 public class KthLargestElementInArray {
 
@@ -26,11 +27,23 @@ public int findKthLargest3(int[] nums, int k) {
     // T(n) = T(n / 2) + n = O(2n)
     // 对比快速排序T(n) = 2T(n / 2) + n = O(nlgn)
     // 区别在于这个被pivot分隔后，只用处理其中的一半，而快排两边都要处理
-    // 耗时18ms
+
+    /**
+     * shuffle非常有必要，性能提升很大
+     */
     public int findKthLargest(int[] nums, int k) {
+        shuffle(nums);
         return findKthLargest(nums, 0, nums.length - 1, k);
     }
 
+    private void shuffle(int[] nums) {
+        Random rnd = new Random(System.currentTimeMillis());
+        for (int i = nums.length - 1; i > 1; i--) {
+            int index = rnd.nextInt(i + 1);
+            swap(nums, index, i);
+        }
+    }
+
     public int findKthLargest(int[] nums, int start, int end, int k) {
         int pivot = partition(nums, start, end);
 
@@ -45,6 +58,10 @@ public int findKthLargest(int[] nums, int start, int end, int k) {
         }
     }
 
+    /**
+     * 这里两端都是闭区间
+     * 这里将数组分为两部分，左边小于pivot，右边大于pivot，中间可能等于pivot
+     */
     public int partition(int[] nums, int start, int end) {
         int pivot = nums[end], left = start, right = end - 1;
 

leetcode/src/Main.java

@@ -4,28 +4,50 @@ public class Main {
 
     public static class Solution {
 
-        class MinStack {
-
-            /** initialize your data structure here. */
-            public MinStack() {
-
-            }
-
-            public void push(int x) {
+        public int findKthLargest(int[] nums, int k) {
+            shuffle(nums);
+            return findKthLargest(nums, 0, nums.length - 1, k);
+        }
 
+        private void shuffle(int[] nums) {
+            Random rnd = new Random(System.currentTimeMillis());
+            for (int i = nums.length - 1; i > 1; i--) {
+                int index = rnd.nextInt(i + 1);
+                swap(nums, index, i);
             }
+        }
 
-            public void pop() {
-
+        public int findKthLargest(int[] nums, int start, int end, int k) {
+            int index = partition(nums, start, end);
+            int count = end - index + 1;
+            if (count < k) {
+                return findKthLargest(nums, start, index - 1, k - count);
+            } else if (count > k) {
+                return findKthLargest(nums, index + 1, end, k);
+            } else {
+                return nums[index];
             }
+        }
 
-            public int top() {
-
+        private int partition(int[] nums, int start, int end) {
+            int left = start, right = end -1, pivot = nums[end];
+            for (int i = start; i < right; ) {
+                if (nums[i] < pivot) {
+                    swap(nums, left++, i++);
+                } else if (nums[i] > pivot) {
+                    swap(nums, right--, i);
+                } else {
+                    i++;
+                }
             }
+            swap(nums, left, end);
+            return left;
+        }
 
-            public int getMin() {
-
-            }
+        private void swap(int[] nums, int i, int j) {
+            int t = nums[i];
+            nums[i] = nums[j];
+            nums[j] = t;
         }
     }