A doubly linked queue is another form of doubly linked list that allows for adding and removing elements, like a stack. However, unlike a stack, where elements are added and removed from the top of the stack, in a queue elements are added to the top of the queue, and removed from the bottom.

FIFO stands for “First In First Out”, and it is the way that data is removed from a queue.

If the Stack can be thought of as a stack of papers, then a queue can be thought of as a line at a bank or at a grocery store checkout.

For a stack of papers, if you wanted to place a new paper on the stack, then you would put it on top of the stack. Then, if you wanted to take a sheet of paper, then you would generally take one from the top of the stack, leaving the bottom of the stack as it was.

For a line of people, anyone new who joins the line will have to stop and wait at the end of the line. Then, when the next person is called from the line, the person who is waiting at the very front of the line will go forward and leave the queue.

Here is what the pseudocode for creating a basic doubly linked queue would look like:

struct/class Obj:
    Data Obj will contain

    prevElement pointer to previous Obj in queue
    nextElement pointer to next Obj in queue

struct/class Queue:
    firstElement pointer to bottom of queue
    lastElement pointer to top of queue

Initialize prevObj
Initialize nextObj

add function (Obj* obj):
    Set prevObj and nextObj to obj's previous element and next element, respectively
    
    if queue is empty:
        Set queue's first element and last element to obj
        Set obj's prev element to NULL
        
    else:
        Set obj's previous element to queue's last element
        Set queue's last element's next element to obj
        Set queue's last element to obj

    Set obj's next element to NULL
    
    Point prevObj and nextObj to each other
    
remove function:
    Grab queue's first element
    Set nextObj to this obj's next element (second in queue)
    
    Set queue's first element to nextObj
    Set nextObj's previous element to NULL
    Set obj's next element to NULL
    
    return obj
    
Main Code:
    Initialize Queue
    
    Initialize elements
    
    Use add and remove functions throughout code