Last element remaining by deleting two largest elements and replacing by their absolute difference if they are unequal

Given an array arr[] of N elements, the task is to perform the following operation: 

• Pick the two largest element from the array and remove these element. If the elements are unequal then insert the absolute difference of the elements into the array.
• Perform the above operations until array has 1 or no element in it. If the array has only one element left then print that element, else print "-1".

Examples: 

Input: arr[] = { 3, 5, 2, 7 } 
Output: 1 
Explanation: 
The two largest elements are 7 and 5. Discard them. Since both are not equal, insert 7 - 5 = 2 into the array. Hence, arr[] = { 3, 2, 2 } 
The two largest elements are 3 and 2. Discard them. Since both are not equal, insert 3 - 2 = 1 into the array. Hence, arr[] = { 1, 2 } 
The two largest elements are 2 and 1. Discard them. Since both are not equal, insert 2 - 1 = 1 into the array. Hence, arr[] = { 1 } 
The only element left is 1. Print the value of the only element left.

Input: arr[] = { 3, 3 } 
Output: -1 
Explanation: 
The two largest elements are 3 and 3. Discard them. Now the array has no elements. So, print -1. 

Approach: To solve the above problem we will use Priority Queue Data Structure. Below are the steps: 

1. Insert all the array elements in the Priority Queue.
2. As priority queue is based on the implementation of Max-Heap. Therefore removing element from it gives the maximum element.
3. Till the size of priority queue is not less than 2, remove two elements(say X & Y) from it and do the following: 
   
   • If X and Y are not same then insert the absolute value of X and Y into the priority queue.
   • Else return to step 3.
4. If the heap has only one element then print that element.
5. Else print "-1".

Below is the implementation of the above approach:  

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;

// Function to print the remaining element
int final_element(int arr[], int n)
{

    // Priority queue can be used
    // to construct max-heap
    priority_queue<int> heap;

    // Insert all element of arr[]
    // into priority queue
    for (int i = 0; i < n; i++)
        heap.push(arr[i]);

    // Perform operation until heap
    // size becomes 0 or 1
    while (heap.size() > 1) {

        // Remove largest element
        int X = heap.top();
        heap.pop();

        // Remove 2nd largest element
        int Y = heap.top();
        heap.pop();

        // If extracted element
        // are not equal
        if (X != Y) {

            // Find X - Y and push
            // it to heap
            int diff = abs(X - Y);
            heap.push(diff);
        }
    }

    // If heap size is 1, then
    // print the remaining element
    if (heap.size() == 1) {

        cout << heap.top();
    }

    // Else print "-1"
    else {
        cout << "-1";
    }
}

// Driver Code
int main()
{
    // Given array arr[]
    int arr[] = { 3, 5, 2, 7 };

    // Size of array arr[]
    int n = sizeof(arr) / sizeof(arr[0]);

    // Function Call
    final_element(arr, n);
    return 0;
}

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

class GFG{ 
    
// Function to print the remaining element     
static public int final_element(Integer[] arr, int n)
{
    if(arr == null)
    {
        return 0;
    }
    
    // Priority queue can be used 
    // to construct max-heap 
    PriorityQueue<Integer> heap = new
    PriorityQueue<Integer>(Collections.reverseOrder()); 
    
    // Insert all element of arr[] 
    // into priority queue 
    for(int i = 0; i < n; i++)
    {
       heap.offer(arr[i]);
    }
    
    // Perform operation until heap 
    // size becomes 0 or 1 
    while (heap.size() > 1)
    {
        
        // Remove largest element 
        int X = heap.poll();
        
        // Remove 2nd largest element 
        int Y = heap.poll();
        
        // If extracted element 
        // are not equal 
        if (X != Y)
        {
            // Find X - Y and push 
            // it to heap 
            int diff = Math.abs(X - Y);
            heap.offer(diff);
        }
    }
    
    // If heap size is 1, then 
    // print the remaining element 
    // Else print "-1" 
    return heap.size() == 1 ? heap.poll() : -1;
}

// Driver code
public static void main (String[] args) 
{ 
    // Given array arr[] 
    Integer arr[] = new Integer[] { 3, 5, 2, 7};
    
    // Size of array arr[] 
    int n = arr.length;
    
    // Function Call 
    System.out.println(final_element(arr, n));  
}
}

// This code is contributed by deepika_sharma

# Python3 program for the above approach 
from queue import PriorityQueue

# Function to print the remaining element
def final_element(arr, n):
    
    # Priority queue can be used
    # to construct max-heap
    heap = PriorityQueue()
    
    # Insert all element of
    # arr[] into priority queue.
    # Default priority queue in Python 
    # is min-heap so use -1*arr[i]
    for i in range(n):
        heap.put(-1 * arr[i])
    
    # Perform operation until heap
    # size becomes 0 or 1
    while (heap.qsize() > 1):

        # Remove largest element
        X = -1 * heap.get()

        # Remove 2nd largest element
        Y = -1 * heap.get()

        # If extracted elements
        # are not equal
        if (X != Y):

            # Find X - Y and push
            # it to heap
            diff = abs(X - Y)
            heap.put(-1 * diff)

    # If heap size is 1, then 
    # print the remaining element
    if (heap.qsize() == 1):
        print(-1 * heap.get())

    # Else print "-1"
    else:
        print("-1")

# Driver Code
if __name__ == '__main__':
    
    # Given array arr[]
    arr = [ 3, 5, 2, 7 ]

    # Size of array arr[]
    n = len(arr)

    # Function call
    final_element(arr, n)

# This code is contributed by himanshu77

// C# program for the above approach 
using System;
using System.Collections.Generic;

class GFG{
    
// Function to print the remaining element 
static void final_element(int[] arr, int n) 
{ 
    
    // Priority queue can be used 
    // to construct max-heap 
    List<int> heap = new List<int>();
  
    // Insert all element of arr[] 
    // into priority queue 
    for(int i = 0; i < n; i++) 
        heap.Add(arr[i]); 
  
    // Perform operation until heap 
    // size becomes 0 or 1 
    while (heap.Count > 1)
    { 
        
        // Remove largest element 
        heap.Sort();
        heap.Reverse();
        int X = heap[0]; 
        heap.RemoveAt(0); 
  
        // Remove 2nd largest element 
        int Y = heap[0]; 
        heap.RemoveAt(0); 
  
        // If extracted element 
        // are not equal 
        if (X != Y) 
        { 
            
            // Find X - Y and push 
            // it to heap 
            int diff = Math.Abs(X - Y); 
            heap.Add(diff); 
        } 
    } 
  
    // If heap size is 1, then 
    // print the remaining element 
    if (heap.Count == 1) 
    { 
        heap.Sort();
        heap.Reverse();
        Console.Write(heap[0]); 
    } 
  
    // Else print "-1" 
    else
    { 
        Console.Write(-1); 
    } 
} 

// Driver code
static void Main() 
{
    
    // Given array arr[] 
    int[] arr = { 3, 5, 2, 7 }; 
    
    // Size of array arr[] 
    int n = arr.Length; 
    
    // Function Call 
    final_element(arr, n);
}
}

// This code is contributed by divyeshrabadiya07

<script>

// Javascript program for the above approach

// Function to print the remaining element
function final_element(arr, n)
{

    // Priority queue can be used
    // to construct max-heap
    var heap = [];

    // Insert all element of arr[]
    // into priority queue
    for (var i = 0; i < n; i++)
        heap.push(arr[i]);
    
    heap.sort((a,b)=>a-b);
    // Perform operation until heap
    // size becomes 0 or 1
    while (heap.length > 1) {

        // Remove largest element
        var X = heap[heap.length-1];
        heap.pop();

        // Remove 2nd largest element
        var Y =  heap[heap.length-1];
        heap.pop();

        // If extracted element
        // are not equal
        if (X != Y) {

            // Find X - Y and push
            // it to heap
            var diff = Math.abs(X - Y);
            heap.push(diff);
        }
        heap.sort((a,b)=>a-b);
    }

    // If heap size is 1, then
    // print the remaining element
    if (heap.length == 1) {
        document.write(heap[heap.length-1]);
    }

    // Else print "-1"
    else {
        document.write("-1");
    }
}

// Driver Code
// Given array arr[]
var arr = [3, 5, 2, 7 ];

// Size of array arr[]
var n = arr.length;

// Function Call
final_element(arr, n);

// This code is contributed by rutvik_56.
</script>

1

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