Question 1

A stack based on a linked list is based on the following code
Class Node{
String element;
Node next;
Node(String el,Node n){
Element = el;
Next = n;
}
}
Node top = null;
The code for implementing the String pop()operation is

Accepted Answer

Option B correctly assigns the next node as the new top and returns the element of the previous top node. This is the correct implementation of the pop() operation in a stack based on linked list. Option A incorrectly assigns the next node as the new top before returning the element, resulting in a null element being returned. Option C incorrectly sets the top to null, removing all elements from the stack. Option D incorrectly returns the element of the current top node before assigning the next node as the new top, resulting in the stack not being properly updated.

Question 2

A stack based on a linked list is based on the following code
Class Node{
String element;
Node next;
Node(String el,Node n){
Element = el;
Next = n;
}
}
Node top = null;
The code for implementing the String peek()operation is

Accepted Answer

Option C correctly checks if the top element of the stack is not null and returns the element's value. If the top element is null, it throws a runtime exception indicating an empty stack. Option A and B do not check for null top elements correctly or return the proper value. Option D returns the entire node instead of just the value of the top element.

Question 3

A queue based on a linked list uses the following code
Class Node{
String element;
Node next;
Node (String el,Node n){
Element = el;
Next = n;
}
}
Node front,rear;
What is the right code for a constructor for such a linked list class?

Accepted Answer

The constructor should initialize both the front and rear pointers to null, indicating that the queue is currently empty. Choice A is incorrect because it sets the "element" and "next" variables of a Node object, which is not relevant to the constructor for the linked list class. Choice B is incorrect because it simply sets the "front" and "rear" pointers to each other, which does not properly initialize the queue. Choice D is incorrect because it sets the "front" pointer to the "element" variable, which is not a Node object and will not work properly as the head of the queue.

Question 4

In a queue implementation that uses an array of fixed size,

Accepted Answer

In a queue implementation using an array of fixed size, it is necessary to use the array as a circular buffer to avoid wasting space and to efficiently manage the enqueue and dequeue operations. By using a circular buffer, we can overwrite elements that have already been dequeued and reuse the space. This makes the implementation much more efficient than if we were using a regular array where we would need to shift all the elements every time an element is dequeued.

Question 5

A stack based on a linked list is based on the following code
Class Node{
String element;
Node next;
Node(String el,Node n){
Element = el;
Next = n;
}
}
Node top = null;
The code for implementing the operation void push(String x)can be written as

Accepted Answer

The code "top = new Node(x,top);" creates a new Node object with the input string x as its element, and sets its next node reference to the current top node. This effectively adds a new node to the top of the stack.

Question 6

Consider a class that uses the following variables to implement an array-based stack:
String [ ] s = new String[100];
Int top = -1;// Note top == -1 indicates stack is empty
A method that implements a String pop()operation can be written as

Accepted Answer

Choice C correctly checks for an empty stack condition and then removes the top element from the stack, assigns it to a temporary variable, sets the top element to null, decrements the top pointer, and finally returns the temporary variable. This is the correct sequence of steps to implement the pop() operation for an array-based stack.

Choice A also performs the correct steps but in a slightly different order. It first decrements the top pointer, assigns the top element to a temporary variable, sets the top element to null, and finally returns the temporary variable. Although the end result is the same, it is better to null out the top element before decrementing the top pointer to avoid any potential confusion.

Choice B, on the other hand, nulls out the top element before decrementing the top pointer, which would result in popping the wrong element from the stack.

Therefore, choice C is the best option.

Question 7

In an implementation of a stack based on a singly-linked list,it is most efficient to add a new item so that

Accepted Answer

In a stack, we always add the new element at the top, which is the head of the linked list. Therefore, it is most efficient to add a new item with the lowest index of all items in the list, so that we can insert the new item at the head of the list in constant time. Adding an item to the end of the list (i.e., with the highest index) would require iterating over the entire list to find the last item, which would take O(n) time. The fact that the new item is not duplicated or sorted is irrelevant to the efficiency of adding it to the stack.

Question 8

The operation for removing an item from a queue is called

Accepted Answer

Dequeue is the operation for removing an item from a queue. The other options (enqueue, extract, disqueue) refer to adding an item, removing an item (not from a queue), and are not commonly used terms, respectively.

Question 9

Consider a class that uses the following variables to implement an array-based stack:
String [ ] s = new String[100];
Int top = -1;// Note top == -1 indicates stack is empty
A method that implements a String peek()operation can be written as

Accepted Answer

This method checks if the stack is empty by checking if the top index is greater than -1 (i.e. not equal to -1). If it is not empty, it returns the element at the top index. If it is empty, it throws a RuntimeException with the message "Empty Stack". This is the best choice because it correctly handles both cases of the stack being empty or not empty without relying on manipulating the top index. Option A is incorrect because it returns the element at index top-1 instead of top, which is not necessarily the correct element. Option C is incorrect because it decrements the top index before returning the element, which can lead to the incorrect element being returned. Option D is incorrect because it checks if top is equal to 0 instead of -1, and also decrements top before returning the element.

Question 10

A queue based on a linked list uses the following code
Class Node{
String element;
Node next;
Node (String el,Node n){
Element = el;
Next = n;
}
}
Node front = null,rear = null;
What is the right code for void add(String x)operation? Such an operation adds x to the queue

Accepted Answer

The correct code for adding an element to a queue based on a linked list is to create a new node with the given element and set its next pointer to null. If the rear pointer is not null, then set the next pointer of the current rear node to the new node and update the rear pointer to the new node. If the rear pointer is null, then set both the front and rear pointers to the new node. Therefore, choice C is the correct code for adding an element to the queue.

Question 11

A queue invariant is a condition

Accepted Answer

A queue invariant is a condition that must be true after the execution of each queue class method. It is a property that a queue must maintain throughout its lifetime. This ensures the correctness and integrity of the data in the queue. Therefore, option B is the correct answer. The other options are not true about queue invariants.

Question 12

The operation for adding an item to a queue is called

Accepted Answer

The correct operation for adding an item to a queue is called "enqueue". Therefore, option B is the best choice. Option A ("requeue") is not a recognized operation in a queue. Option C ("dequeue") is the operation to remove an item from the front of the queue. Option D ("proqueue") is not a recognized term in queue operations.

Question 13

Which of the following operations is not a stack operation?

Accepted Answer

A stack is a data structure that follows the Last In, First Out (LIFO) principle, allowing operations only at one end: push (add an item) and pop (remove the top item). The operations described in A, B, and C do not adhere to this principle, as they involve accessing or modifying elements at arbitrary positions or specific values, which are not standard stack operations.

Question 14

A queue based on a linked list uses the following code
Class Node{
String element;
Node next;
Node (String el,Node n){
Element = el;
Next = n;
}
}
Node front = null,rear = null;
What is the right code for the boolean empty()method?

Accepted Answer

The correct code for the boolean empty() method in a queue based on a linked list is to simply check if the front pointer is null. If it is null, then the queue is empty; otherwise, it is not empty. So, option A is the correct choice.

Option B is incorrect because it is checking if the rear and front pointers are the same, which does not necessarily mean that the queue is empty.

Option C is also incorrect because it is throwing a runtime exception if the front pointer is null, which is unnecessary for simply checking if the queue is empty.

Option D is incorrect because it is checking if the front and rear pointers are equal, which may not be the case when the queue is empty.

Question 15

A queue based on a linked list uses the following code
Class Node{
String element;
Node next;
Node (String el,Node n){
Element = el;
Next = n;
}
}
Node front = null,rear = null;
What is the right code for String remove()operation? Such an operation removes and returns an element from the queue.

Accepted Answer

The correct code checks if the queue is empty by verifying if 'front' is null. It then stores the front element, updates 'front' to the next node, and if the queue becomes empty after removal, it sets 'rear' to null.

Question 16

A stack based on a linked list is based on the following code
Class Node{
String element;
Node next;
Node(String el,Node n){
Element = el;
Next = n;
}
}
Node top = null;
The code for testing whether the stack is empty is

Accepted Answer

The code simply needs to check whether the top node is null or not to determine if the stack is empty. Choice A correctly checks for this condition and returns a boolean value accordingly. The other choices either have syntax errors (B) or unnecessary code (C, D).

Quiz 21: Stacks and Queues

A stack based on a linked list is based on the following code Class Node{ String element; Node next; Node(String el,Node n) { Element = el; Next = n; } } Node top = null; The code for implementing the String pop() operation is

A stack based on a linked list is based on the following code Class Node{ String element; Node next; Node(String el,Node n) { Element = el; Next = n; } } Node top = null; The code for implementing the String peek() operation is

A queue based on a linked list uses the following code Class Node{ String element; Node next; Node (String el,Node n) { Element = el; Next = n; } } Node front,rear; What is the right code for a constructor for such a linked list class?

In a queue implementation that uses an array of fixed size,

A stack based on a linked list is based on the following code Class Node{ String element; Node next; Node(String el,Node n) { Element = el; Next = n; } } Node top = null; The code for implementing the operation void push(String x) can be written as

Consider a class that uses the following variables to implement an array-based stack: String [ ] s = new String[100]; Int top = -1;// Note top == -1 indicates stack is empty A method that implements a String pop() operation can be written as

In an implementation of a stack based on a singly-linked list,it is most efficient to add a new item so that

The operation for removing an item from a queue is called

Consider a class that uses the following variables to implement an array-based stack: String [ ] s = new String[100]; Int top = -1;// Note top == -1 indicates stack is empty A method that implements a String peek() operation can be written as

A queue based on a linked list uses the following code Class Node{ String element; Node next; Node (String el,Node n) { Element = el; Next = n; } } Node front = null,rear = null; What is the right code for void add(String x) operation? Such an operation adds x to the queue

A queue invariant is a condition

The operation for adding an item to a queue is called

Which of the following operations is not a stack operation?

A queue based on a linked list uses the following code Class Node{ String element; Node next; Node (String el,Node n) { Element = el; Next = n; } } Node front = null,rear = null; What is the right code for the boolean empty() method?

A queue based on a linked list uses the following code Class Node{ String element; Node next; Node (String el,Node n) { Element = el; Next = n; } } Node front = null,rear = null; What is the right code for String remove() operation? Such an operation removes and returns an element from the queue.

A stack based on a linked list is based on the following code Class Node{ String element; Node next; Node(String el,Node n) { Element = el; Next = n; } } Node top = null; The code for testing whether the stack is empty is