Final answer:
An input-restricted dequeue can operate as both a queue and a stack because it allows insertions at one end but deletions from both ends, enabling FIFO and LIFO operations respectively.
Step-by-step explanation:
An input-restricted dequeue is a data structure where insertion is restricted to one end but deletion can occur at either end. This structure can function as both a queue and a stack. When operating as a queue, elements can be enqueued at the back and dequeued from the front. While as a stack, elements can be pushed (enqueued) and popped (dequeued) from the same end. Therefore, it can be utilized differently depending on the operations allowed.
In this case, you can use the input-restricted dequeue as a queue since elements are added at the rear and can be removed from the front, maintaining the first-in-first-out (FIFO) order. Similarly, you can use it as a stack, with the same end for insertion and deletion, thereby maintaining the last-in-first-out (LIFO) order.
For example, if we have a restricted dequeue with the elements A, B, and C arranged in that order, we can insert an element D at the front, remove an element from the front (which would be D), insert an element E at the back, and remove an element from the back (which would be E). We can also perform these operations in a different order and still maintain the functionality of a queue or a stack.
Therefore, the correct answer is option d: both as a queue and as a stack.