Leetcode 215: Kth Largest Element in an Array

Posted 2020-10-16 Keep walking

Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

For example,
Given [3,2,1,5,6,4] and k = 2, return 5.

Note: 
You may assume k is always valid, 1 ≤ k ≤ array‘s length.

 

 1 public class Solution {
 2     public int FindKthLargest(int[] nums, int k) {        
 3         return QuickSelect(nums, 0, nums.Length - 1, nums.Length - k);
 4     }
 5     
 6     private int QuickSelect(int[] nums, int start, int end, int k)
 7     {
 8         if (start >= end) return nums[start];
 9         
10         int i = start, j = start, pivot = nums[end];
11         
12         while (j < end)
13         {
14             if (nums[j] < pivot)
15             {
16                 if (i != j)
17                 {
18                     var tmp = nums[j];
19                     nums[j] = nums[i];
20                     nums[i] = tmp;
21                 }
22                 
23                 i++;
24             }
25             
26             j++;
27         }
28         
29         if (i != end)
30         {
31             nums[end] = nums[i];
32             nums[i] = pivot;
33         }
34         
35         if (k == i - start)
36         {
37             return nums[i];
38         }
39         else if (k > i - start)
40         {
41             return QuickSelect(nums, i + 1, end, k - i + start - 1);     
42         }
43         else
44         {
45             return QuickSelect(nums, start, i - 1, k);
46         }
47     }
48 }