How to Reverse a Deque in C++?

In C++ STL, we have a container called deque(short for double-ended queue) that allows fast insertion and deletion operations at both the beginning and end. In this article, we will learn how to reverse a deque in C++.

Example:

Input: 
myDeque = {1, 2, 3, 4, 5};

Output:
Reversed Deque: 5 4 3 2 1

Reverse Deque in C++ STL

To reverse a std::deque in C++,  we can use the std::reverse() function from the <algorithm> library that reverses the order of elements in a given range. We need to pass the beginning and end iterators of the deque to this function.

Syntax of std::reverse()

reverse(first, last);

Here,

• first is the iterator to the beginning of the deque.
• last is the iterator to the end of the deque.

C++ Program to Reverse a Deque

The below example demonstrates how we can reverse a deque in C++.

C++

// C++ Program to demonstrate how we can reverse a deque
#include <algorithm>
#include <deque>
#include <iostream>
using namespace std;

int main()
{

    // Creating a deque
    deque<int> dq = { 1, 2, 3, 4, 5 };

    // Printing the original deque
    cout << "Original Deque: ";
    for (int num : dq) {
        cout << num << " ";
    }
    cout << endl;

    // Reversing the deque
    reverse(dq.begin(), dq.end());

    // Printing the reversed deque
    cout << "Reversed Deque: ";
    for (int num : dq) {
        cout << num << " ";
    }
    cout << endl;

    return 0;
}

Output

Original Deque: 1 2 3 4 5 
Reversed Deque: 5 4 3 2 1 

Time Complexity: O(N), here N is the number of elements in deque.
Auxiliary Space: O(1)