What is the opposite of a stack? This may be a weird question. However, a stack follows LIFO, last in first out. The opposite of that is first-in-first-out (FIFO). So, in some sense, a FIFO ADT can be considered as the opposite of a stack. This is not very different from a queue of people waiting for a bus or at a doctor's clinic. The first person to show up gets the first chance to get onto the bus or to get to see the doctor. The second person gets the second chance. No wonder, such an abstract data type is called a queue. Appending to the end of a queue is called enqueuing and removing from it is called dequeuing. The contract is, of course, that the first value that is enqueued would be the first to be dequeued. The following figure illustrates this operation:
The queue ADT has the following operations:
The...