Median of Stream of Running Integers using STL | Set 2

Given an array arr[] of size N representing integers required to be read as a data stream, the task is to calculate and print the median after reading every integer.

Examples:

Input: arr[] = { 5, 10, 15 }
Output: 5 7.5 10
Explanation:
After reading arr[0] from the data stream, the median is 5.
After reading arr[1] from the data stream, the median is 7.5.
After reading arr[2] from the data stream, the median is 10.

Input: arr[] = { 1, 2, 3, 4 }
Output: 1 1.5 2 2.5

Approach: The problem can be solved using Ordered Set. Follow the steps below to solve the problem:

• Initialize a multi Ordered Set say, mst to store the array elements in a sorted order.
• Traverse the array using variable i. For every i^th element insert arr[i] into mst and check if the variable i is even or not. If found to be true then print the median using (*mst.find_by_order(i / 2)).
• Otherwise, print the median by taking the average of (*mst.find_by_order(i / 2)) and (*mst.find_by_order((i + 1) / 2)).

Below is the implementation of the above approach:

C++

// C++ program to implement
// the above approach

#include <iostream>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type,
less_equal<int>, rb_tree_tag,
tree_order_statistics_node_update> idxmst;



// Function to find the median
// of running integers
void findMedian(int arr[], int N)
{
    // Initialise a multi ordered set
    // to store the array elements
    // in sorted order
    idxmst mst;

    // Traverse the array
    for (int i = 0; i < N; i++) {

        // Insert arr[i] into mst
        mst.insert(arr[i]);

        // If i is an odd number
        if (i % 2 != 0) {

            // Stores the first middle
            // element of mst
            double res
             = *mst.find_by_order(i / 2);

            // Stores the second middle
            // element of mst
            double res1
              = *mst.find_by_order(
                             (i + 1) / 2);

            cout<< (res + res1) / 2.0<<" ";
        }
        else {

            // Stores middle element of mst
            double res
               = *mst.find_by_order(i / 2);

            // Print median
            cout << res << " ";
        }
    }
}

// Driver Code
int main()
{
    // Given stream of integers
    int arr[] = { 1, 2, 3, 3, 4 };

    int N = sizeof(arr) / sizeof(arr[0]);

    // Function call
    findMedian(arr, N);
}

Time Complexity: O(N * log(N))
Auxiliary Space: O(N)