KthElementInArray.java

package com.interview.array;

import java.util.Arrays;

/**
 * Kth largest element in an array.
 * Use quickselect of quicksort to find the solution in hopefully O(nlogn) time.
 * Test cases
 * Sorted array
 * Reverse sorted array
 */
public class KthElementInArray {
    
    public int kthElement(int arr[],int k){
        int low = 0;
        int high = arr.length-1;
        int pos =0;
        while(true){
            pos = quickSelect(arr,low,high);
            if(pos == (k+low)){
                return arr[pos];
            }
            if(pos > k + low){
                high = pos-1;
            }else{
                k = k - (pos - low + 1);
                low = pos+1;
            }
        }
    }
    
    private int quickSelect(int arr[],int low,int high){
        int pivot = low;
        low++;
        while(low <= high){
            if(arr[pivot] > arr[low]){
                low++;
                continue;
            }
            if(arr[pivot] <= arr[high]){
                high--;
                continue;
            }
            swap(arr,low,high);
        }
        if(arr[high] < arr[pivot]){
            swap(arr,pivot,high);
        }
        return high;
    }
    
    private void swap(int arr[],int low,int high){
        int temp = arr[low];
        arr[low] = arr[high];
        arr[high] = temp;
    }
    
    public static void main(String args[]){
        int arr[] = {6, 2, 1, 6, 8, 9, 6};
        KthElementInArray kthElement = new KthElementInArray();
        System.out.print(kthElement.kthElement(arr, arr.length/2));
        System.out.print(Arrays.toString(arr));
    }
    
}