# Data Structure Questions and Answers – Double Ended Queue (Dequeue)

This set of Data Structure Multiple Choice Questions & Answers (MCQs) focuses on “Double Ended Queue (Dequeue)”.

1. What is a dequeue?
a) A queue with insert/delete defined for both front and rear ends of the queue
b) A queue implemented with a doubly linked list
c) A queue implemented with both singly and doubly linked lists
d) A queue with insert/delete defined for front side of the queue

Explanation: A dequeue or a double ended queue is a queue with insert/delete defined for both front and rear ends of the queue.

2. Select the function which performs insertion at the front end of the dequeue?
a)

```public void function(Object item)
{
Node temp = new Node(item,null);
if(isEmpty())
{
temp.setNext(trail);
}
else
{
temp.setNext(cur);
}
size++;
}```

b)

```public void function(Object item)
{
Node temp = new Node(item,null);
if(isEmpty())
{
temp.setNext(trail);
}
else
{
temp.setNext(cur);
}
size++;
}```

c)

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```public void function(Object item)
{
Node temp = new Node(item,null);
if(isEmpty())
{
temp.setNext(cur);
}
else
{
temp.setNext(trail);
}
size++;
}```

d)

```public void function(Object item)
{
Node temp = new Node(item,null);
if(isEmpty())
{
temp.setNext(cur);
cur.setNext(temp);
}
else
{
trail.setNext(temp);
}
size++;
}```
Explanation: Create a new node, if the current list is empty, the ‘head’ points to this node and this new node points to ‘trail’. Otherwise, ‘head’ points to the new node and this in turn points to the current first element(head.getNext()).

3. What is the functionality of the following piece of code?

```public void function(Object item)
{
Node temp=new Node(item,trail);
if(isEmpty())
{
temp.setNext(trail);
}
else
{
while(cur.getNext()!=trail)
{
cur=cur.getNext();
}
cur.setNext(temp);
}
size++;
}```

a) Insert at the front end of the dequeue
b) Insert at the rear end of the dequeue
c) Fetch the element at the rear end of the dequeue
d) Fetch the element at the front end of the dequeue

Explanation: If the list is empty, this new node will point to ‘trail’ and will be pointed at by ‘head’. Otherwise, traverse till the end of the list and insert the new node there.

4. What are the applications of dequeue?
a) A-Steal job scheduling algorithm
b) Can be used as both stack and queue
c) To find the maximum of all sub arrays of size k
d) All of the mentioned

Explanation: All of the mentioned can be implemented with a dequeue.

5. Which of the following can be used to delete an element from the front end of the queue?
a)

```public Object deleteFront() throws emptyDEQException
{
if(isEmpty())
throw new emptyDEQException("Empty");
else
{
Node cur = temp;
Object e = temp.getEle();
size--;
return e;
}
}```

b)

```public Object deleteFront() throws emptyDEQException
{
if(isEmpty())
throw new emptyDEQException("Empty");
else
{
Node cur = temp.getNext();
Object e = temp.getEle();
size--;
return e;
}
}```

c)

```public Object deleteFront() throws emptyDEQException
{
if(isEmpty())
throw new emptyDEQException("Empty");
else
{
Node cur = temp.getNext();
Object e = temp.getEle();
size--;
return e;
}
}```

d)

```public Object deleteFront() throws emptyDEQException
{
if(isEmpty())
throw new emptyDEQException("Empty");
else
{
Node cur = temp.getNext();
Object e = temp.getEle();
temp.setNext(cur);
size--;
return e;
}
}```
Explanation: Have two pointers, one(temp) pointing to the first element and the other(cur) pointing to the second element. Make the ‘head’ point to the second element, this removes all reference for ‘temp’.

6. Which of the following can be used to delete an element from the rear end of the queue?
a)

```public Object deleteRear() throws emptyDEQException
{
if(isEmpty())
throw new emptyDEQException("Empty");
else
{
Node cur = temp;
while(temp.getNext() != trail)
{
temp = temp.getNext();
cur = cur.getNext();
}
Object e = temp.getEle();
cur.setNext(trail);
size--;
return e;
}
}```

b)

```public Object deleteRear() throws emptyDEQException
{
if(isEmpty())
throw new emptyDEQException("Empty");
else
{
while(temp != trail)
{
temp = temp.getNext();
cur = cur.getNext();
}
Object e = temp.getEle();
cur.setNext(trail);
size--;
return e;
}
}```

c)

```public Object deleteRear() throws emptyDEQException
{
if(isEmpty())
throw new emptyDEQException("Empty");
else
{
while(temp.getNext()!=trail)
{
temp = temp.getNext();
cur = cur.getNext();
}
Object e = temp.getEle();
cur.setNext(trail);
size--;
return e;
}
}```

d)

```public Object deleteRear() throws emptyDEQException
{
if(isEmpty())
throw new emptyDEQException("Empty");
else
{
while(temp.getNext()!=trail)
{
temp = temp.getNext();
cur = cur.getNext();
}
Object e = temp.getEle();
temp.setNext(trail);
size--;
return e;
}
}```
Explanation: Traverse till the end of the list with a pointer ‘temp’ and another ‘cur’ which is trailing behind temp, make ‘cur’ point to trail, this removes all reference for ‘temp’.

7. What is the time complexity of deleting from the rear end of the dequeue implemented with a singly linked list?
a) O(nlogn)
b) O(logn)
c) O(n)
d) O(n2)
Explanation: Since a singly linked list is used, first you have to traverse till the end, so the complexity is O(n).

8. After performing these set of operations, what does the final list look contain?

```InsertFront(10);
InsertFront(20);
InsertRear(30);
DeleteFront();
InsertRear(40);
InsertRear(10);
DeleteRear();
InsertRear(15);
display();```

a) 10 30 10 15
b) 20 30 40 15
c) 20 30 40 10
d) 10 30 40 15

Explanation: A careful tracing of the given operation yields the result.
10
20 10
20 10 30
10 30
10 30 40
10 30 40 10
10 30 40
10 30 40 15

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