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Efficient way to initialize a priority queue

  • Last Updated : 12 Aug, 2021

STL Priority Queue is the implementation of Heap Data Structure. By default, it’s a max heap, and can be easily for primitive data types. There are some important applications of it which can be found in this article.

Priority queue can be initialized in two ways either by pushing all elements one by one or by initializing using their constructor. In this article, we will discuss both methods and examine their time complexities.

Method 1: The simplest approach is to traverse the given array and push each element one by one in the priority queue. In this method, the push method in the priority queue takes O(log N) time. Where N is the number of elements in the array.

Below is the implementation of the above approach:

C++




// C++ program to initialize the
// priority queue
#include <bits/stdc++.h>
using namespace std;
 
// Driver Code
int main()
{
    int arr[] = { 15, 25, 6, 54, 45, 26, 12 };
    int N = sizeof(arr) / sizeof(arr[0]);
 
    // Initialize priority_queue
    priority_queue<int> pq;
 
    // Traverse the array arr[]
    for (int i = 0; i < N; i++) {
 
        // Push the element arr[i]
        pq.push(arr[i]);
    }
 
    cout << "The elements in priority"
         << " Queue are: ";
 
    // Traverse until pq is non-empty
    while (!pq.empty()) {
 
        // Print the element in pq
        cout << pq.top() << " ";
 
        // Pop the top element
        pq.pop();
    }
 
    return 0;
}


Java




// Java program to initialize the
// priority queue
import java.util.*;
public class GFG
{
    public static void main(String[] args)
    {
        int[] arr = { 15, 25, 6, 54, 45, 26, 12 };
        int N = arr.length;
      
        // Initialize priority_queue
        Vector<Integer> pq = new Vector<Integer>();
      
        // Traverse the array arr[]
        for (int i = 0; i < N; i++)
        {
      
          // Push the element arr[i]
          pq.add(arr[i]);
        }
        Collections.sort(pq);
        Collections.reverse(pq);
        System.out.print("The elements in priority" + " Queue are: ");
      
        // Traverse until pq is non-empty
        while (pq.size() > 0)
        {
      
          // Print the element in pq
          System.out.print(pq.get(0) + " ");
      
          // Pop the top element
          pq.remove(0);
        }
    }
}
 
// This code is contributed by divyesh072019.


Python3




# Python3 program to initialize the
# priority queue
 
# Driver Code
if __name__ == '__main__':
    arr = [15, 25, 6, 54, 45, 26, 12]
    N = len(arr)
 
    # Initialize priority_queue
    pq = []
 
    # Traverse the array arr[]
    for i in range(N):
       
        # Push the element arr[i]
        pq.append(arr[i])
    print("The elements in priority Queue are: ", end = "")
    pq = sorted(pq)
 
    # Traverse until pq is non-empty
    while (len(pq) > 0):
 
        # Print the element in pq
        print(pq[-1], end = " ")
 
        # Pop the top element
        del pq[-1]
 
        # This code is contributed by mohit kumar 29.


C#




// C# program to initialize the
// priority queue
using System;
using System.Collections.Generic;
class GfG
{
  public static void Main()
  {
    int[] arr = { 15, 25, 6, 54, 45, 26, 12 };
    int N = arr.Length;
 
    // Initialize priority_queue
    List<int> pq = new List<int>();
 
    // Traverse the array arr[]
    for (int i = 0; i < N; i++) {
 
      // Push the element arr[i]
      pq.Add(arr[i]);
    }
 
    pq.Sort();
    pq.Reverse();
 
    Console.Write("The elements in priority" + " Queue are: ");
 
    // Traverse until pq is non-empty
    while (pq.Count > 0) {
 
      // Print the element in pq
      Console.Write(pq[0] + " ");
 
      // Pop the top element
      pq.RemoveAt(0);
    }
  }
}
 
// This code is contributed by divyeshrabadiya07.


Javascript




<script>
    // Javascript program to initialize the priority queue
     
    let arr = [ 15, 25, 6, 54, 45, 26, 12 ];
    let N = arr.length;
  
    // Initialize priority_queue
    let pq = [];
  
    // Traverse the array arr[]
    for (let i = 0; i < N; i++) {
  
      // Push the element arr[i]
      pq.push(arr[i]);
    }
  
    pq.sort(function(a, b){return a - b});
    pq.reverse();
  
    document.write("The elements in priority" + " Queue are: ");
  
    // Traverse until pq is non-empty
    while (pq.length > 0) {
  
      // Print the element in pq
      document.write(pq[0] + " ");
  
      // Pop the top element
      pq.shift();
    }
 
// This code is contributed by suresh07.
</script>


Output: 



The elements in priority Queue are: 54 45 26 25 15 12 6

 

Time Complexity: O(N*log N),  where N is the total number of elements in the array.
Auxiliary Space: O(N)

Method 2: In this method, copy all the array elements into the priority queue while initializing it (this copying will be happened using the copy constructor of priority_queue). In this method, the priority_queue will use the build heap method internally. So the build heap method is taking O(N) time.

Syntax:

priority_queue<int> pq(address of the first element, address of the next of the last element);

Syntax for the array:

priority_queue<int> pq (arr, arr + N)
where arr is the array and N is the size of the array.

Syntax for the vector:

priority_queue<int> pq(v.begin(), v.end()); 
 where v is the vector.

Below is the implementation of the above approach:

C++




// C++ program to initialize the
// priority queue
#include <iostream>
#include <queue>
using namespace std;
 
// Driver Code
int main()
{
    int arr[] = { 15, 25, 6, 54, 45, 26, 12 };
    int N = sizeof(arr) / sizeof(arr[0]);
 
    // By this type of initialization
    // the priority_queue is using
    // build heap to make the max heap
    cout << "The elements in priority"
         << " Queue are: ";
 
    // Initialize priority_queue
    priority_queue<int> pq(arr, arr + N);
 
    // Iterate until pq is non empty
    while (!pq.empty()) {
 
        // Print the element
        cout << pq.top() << " ";
        pq.pop();
    }
 
    return 0;
}


Python3




# Python3 program to initialize the
# priority queue
 
# Driver Code
if __name__=='__main__':
     
    arr = [ 15, 25, 6, 54, 45, 26, 12 ]
    N = len(arr)
     
    # By this type of initialization
    # the priority_queue is using
    # build heap to make the max heap
    print("The elements in priority Queue are: ", end = '')
     
    # Initialize priority_queue
    pq = arr
    pq.sort()
 
    # Iterate until pq is non empty
    while (len(pq) != 0):
       
        # Print the element
        print(pq[-1], end = ' ')
        pq.pop()
     
    # This code is contributed by rutvik_56.


Javascript




<script>
// JAvaScript program to initialize the
// priority queue
let arr = [ 15, 25, 6, 54, 45, 26, 12 ];
let N = arr.length;
 
// By this type of initialization
// the priority_queue is using
// build heap to make the max heap
document.write("The elements in priority Queue are: ")
 
// Initialize priority_queue
let pq = arr;
pq.sort(function(a, b){return a - b;});
 
// Iterate until pq is non empty
while(pq.length != 0)
 
    // Print the element
    document.write(pq.pop()+" ");
 
// This code is contributed by unknown2108
</script>


Output: 

The elements in priority Queue are: 54 45 26 25 15 12 6

 

Time Complexity: O(N), where N is the total number of elements in the array.
Auxiliary Space: O(N)

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