Kth Largest Element in unsorted Array.

KthLargest.java

public class KthLargest{
  
  public int findKthLargestPQ(int[] nums, int k) {
    
    PriorityQueue<Integer> pq = new PriorityQueue<Integer>();
 
    int n = nums.length;
 
    for(int i = 0; i < n; ++i){
       
    if(pq.size() == k){
      if(pq.peek() < nums[i]){
        pq.remove();
        pq.add(nums[i]);
      }
    }
       
    else
      pq.add(nums[i]);
   }
   
   return pq.remove();
}
  
  
  public int findKthLargestQuickSelect(int[] nums, int k) {
     
        k = nums.length - k;
        
        int lo = 0;
        int hi = nums.length  - 1;
        
        while(hi > lo){
            
            int pivot = quickSelect(nums, lo, hi);
            
            if(pivot < k){
                lo = pivot + 1;
            }
            
            else if(pivot > k){
                hi = pivot - 1;
            }
            
            else return nums[pivot];
        }
        
        return nums[lo];
    }
    
    private int quickSelect(int[] ar, int lo, int hi){
        
        int pivot = lo;
        
        int i = lo;
        int j = hi + 1;
        
        while(true){
            
            while(ar[++i] < ar[pivot]) if(i == hi) break;
            while(ar[--j] > ar[pivot]) if(j == lo) break;
        
            if(i >= j) break;
            
            swap(ar, i , j);
        }
        
        swap(ar, j, pivot);
        
        return j;
    }
    
    private void swap(int[] ar, int x, int y){
        int temp = ar[x];
        ar[x] = ar[y];
        ar[y] = temp;
    }
}