Queue

1. Overview

• linear data structure
• follows the First-In-First-Out (FIFO) principle
• elements are added to the rear and removed from the front
• C++ Standard Template Library (STL) provides the std::queue class for implementation

2. Operations

Operation	Time Complexity	Description
Enqueue	O(1)	Adds an element to the rear.
Dequeue	O(1)	Removes an element from the front.
Front	O(1)	Returns the front element.
Back	O(1)	Returns the rear element.
Empty	O(1)	Checks if the queue is empty.
Size	O(1)	Returns the size of the queue.

Note STL queue internally keeps a counter to track its number of elements, which allows for the size() function to be constant

3. STL Implementation

int main() {
    queue<int> q;
 
    // Enqueue elements
    q.push(10);
    q.push(20);
    q.push(30);
 
    cout << "Front element: " << q.front() << endl;  // Output: 10
    cout << "Back element: " << q.back() << endl;    // Output: 30
 
    // Dequeue elements
    q.pop();
    cout << "After one dequeue, front: " << q.front() << endl;  // Output: 20
 
    // Check if the queue is empty
    cout << "Is the queue empty? " << (q.empty() ? "Yes" : "No") << endl;
 
    // Print the size of the queue
    cout << "Queue size: " << q.size() << endl;
 
    return 0;
}