Java Program to Find the K'th largest element in a stream

Given an infinite stream of integers, find the k'th largest element at any point of time.
Example: 

Input:
stream[] = {10, 20, 11, 70, 50, 40, 100, 5, ...}
k = 3

Output:    {_,   _, 10, 11, 20, 40, 50,  50, ...}

Extra space allowed is O(k). 

We have discussed different approaches to find k'th largest element in an array in the following posts. 
K’th Smallest/Largest Element in Unsorted Array | Set 1 
K’th Smallest/Largest Element in Unsorted Array | Set 2 (Expected Linear Time)
K’th Smallest/Largest Element in Unsorted Array | Set 3 (Worst Case Linear Time)
Here we have a stream instead of whole array and we are allowed to store only k elements.

A Simple Solution is to keep an array of size k. The idea is to keep the array sorted so that the k'th largest element can be found in O(1) time (we just need to return first element of array if array is sorted in increasing order) 
How to process a new element of stream? 
For every new element in stream, check if the new element is smaller than current k'th largest element. If yes, then ignore it. If no, then remove the smallest element from array and insert new element in sorted order. Time complexity of processing a new element is O(k).

A Better Solution is to use a Self Balancing Binary Search Tree of size k. The k'th largest element can be found in O(Logk) time. 
How to process a new element of stream? 
For every new element in stream, check if the new element is smaller than current k'th largest element. If yes, then ignore it. If no, then remove the smallest element from the tree and insert new element. Time complexity of processing a new element is O(Logk).

An Efficient Solution is to use Min Heap of size k to store k largest elements of stream. The k'th largest element is always at root and can be found in O(1) time. 
How to process a new element of stream? 
Compare the new element with root of heap. If new element is smaller, then ignore it. Otherwise replace root with new element and call heapify for the root of modified heap. Time complexity of finding the k'th largest element is O(Logk).
 

// Java program for the above approach
import java.io.*;
import java.util.*;

class GFG {
  
  /*
  using min heap DS

  how data are stored in min Heap DS
         1
       2   3
  if k==3 , then top element of heap
  itself the kth largest largest element

  */
  static PriorityQueue<Integer> min;
  static int k;

  static List<Integer> getAllKthNumber(int arr[])
  {
    
    // list to store kth largest number
    List<Integer> list = new ArrayList<>();

    // one by one adding values to the min heap
    for (int val : arr) {

      // if the heap size is less than k , we add to
      // the heap
      if (min.size() < k)
        min.add(val);

      /*
      otherwise ,
      first we  compare the current value with the
      min heap TOP value

      if TOP val > current element , no need to
      remove TOP , bocause it will be the largest kth
      element anyhow

      else  we need to update the kth largest element
      by removing the top lowest element
      */

      else {
        if (val > min.peek()) {
          min.poll();
          min.add(val);
        }
      }

      // if heap size >=k we add 
      // kth largest element
      // otherwise -1

      if (min.size() >= k)
        list.add(min.peek());
      else
        list.add(-1);
    }
    return list;
  }

  // Driver Code
  public static void main(String[] args)
  {
    min = new PriorityQueue<>();

    k = 4;

    int arr[] = { 1, 2, 3, 4, 5, 6 };

    List<Integer> res = getAllKthNumber(arr);

    for (int x : res)
      System.out.print(x + " ");
  }

    // This code is Contributed by Pradeep Mondal P
}

Output:

K is 3
Enter next element of stream 23
Enter next element of stream 10
Enter next element of stream 15
K'th largest element is 10
Enter next element of stream 70
K'th largest element is 15
Enter next element of stream 5
K'th largest element is 15
Enter next element of stream 80
K'th largest element is 23
Enter next element of stream 100
K'th largest element is 70
Enter next element of stream
CTRL + C pressed

Time Complexity: O(N*logK)

O(logK) is the time to add/remove an element from the priority queue of size K and in the worst case we need to do this for all values of N, thus the time complexity becomes O(N*logK)

Auxiliary Space: O(N)

Please refer complete article on K'th largest element in a stream for more details!