Leetcode 215 Kth Largest Element in an Array

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.

Solution 1: Sort the array and return nums[k-1], it is O(nlogn) time and O(1) space.

Solution 2: QuickSort partition. O(n) best and O(n^2) worst. Can be shuffle to grantee the performance

 public class Solution {  
   public int findKthLargest(int[] nums, int k) {  
     k--;  
     int lo=0, hi=nums.length-1;  
     while (lo<=hi) {  
       int[] p=partition(nums,lo,hi);  
       if (k>p[1]) lo=p[1]+1;  
       else if (k<p[0]) hi=p[0]-1;  
       else return nums[k];  
     }  
     return 0;  
   }  
   private int[] partition(int[] nums, int lo, int hi) {  
     int v=nums[lo];  
     int j=lo, i=lo+1, k=hi;  
     while (i<=k) {  
       if (nums[i]==v) i++;  
       else if (nums[i]<v) swap(nums,i,k--);  
       else swap(nums,i++,j++);  
     }  
     return new int[]{j,k};  
   }  
   private void swap(int[] nums, int i, int j) {  
     int temp=nums[i];  
     nums[i]=nums[j];  
     nums[j]=temp;  
   }  
 }