Queue
Question 1 
Following is C like pseudo code of a function that takes a Queue as an argument, and uses a stack S to do processing.
void fun(Queue *Q) { Stack S; // Say it creates an empty stack S // Run while Q is not empty while (!isEmpty(Q)) { // deQueue an item from Q and push the dequeued item to S push(&S, deQueue(Q)); } // Run while Stack S is not empty while (!isEmpty(&S)) { // Pop an item from S and enqueue the poppped item to Q enQueue(Q, pop(&S)); } }What does the above function do in general?
Removes the last from Q  
Keeps the Q same as it was before the call  
Makes Q empty  
Reverses the Q 
Discuss it
Question 1 Explanation:
The function takes a queue Q as an argument. It dequeues all items of Q and pushes them to a stack S. Then pops all items of S and enqueues the items back to Q. Since stack is LIFO order, all items of queue are reversed.
Question 2 
Which one of the following is an application of Queue Data Structure?
When a resource is shared among multiple consumers.  
When data is transferred asynchronously (data not necessarily received at same rate as sent) between two processes  
Load Balancing  
All of the above 
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Question 2 Explanation:
See http://www.geeksforgeeks.org/applicationsofqueuedatastructure/ for details.
Question 3 
How many stacks are needed to implement a queue. Consider the situation where no other data structure like arrays, linked list is available to you.
1  
2  
3  
4 
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Question 3 Explanation:
A queue can be implemented using two stacks. See following for implementation.
http://www.geeksforgeeks.org/queueusingstacks/
Question 4 
How many queues are needed to implement a stack. Consider the situation where no other data structure like arrays, linked list is available to you.
1  
2  
3  
4 
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Question 4 Explanation:
A stack can be implemented using two queues. Please see following for details.
http://www.geeksforgeeks.org/implementstackusingqueue/
Question 5 
A priority queue can efficiently implemented using which of the following data structures? Assume that the number of insert and peek (operation to see the current highest priority item) and extraction (remove the highest priority item) operations are almost same.
Array  
Linked List  
Heap Data Structures like Binary Heap, Fibonacci Heap  
None of the above 
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Question 5 Explanation:
Question 6 
Which of the following is true about linked list implementation of queue?
In push operation, if new nodes are inserted at the beginning of linked list, then in pop operation, nodes must be removed from end.  
In push operation, if new nodes are inserted at the end, then in pop operation, nodes must be removed from the beginning.  
Both of the above  
None of the above 
Discuss it
Question 6 Explanation:
To keep the First In First Out order, a queue can be implemented using linked list in any of the given two ways.
Question 7 
Suppose a circular queue of capacity (n – 1) elements is implemented with an array of n elements. Assume that the insertion and deletion operation are carried out using REAR and FRONT as array index variables, respectively. Initially, REAR = FRONT = 0. The conditions to detect queue full and queue empty are
Full: (REAR+1) mod n == FRONT, empty: REAR == FRONT  
Full: (REAR+1) mod n == FRONT, empty: (FRONT+1) mod n == REAR  
Full: REAR == FRONT, empty: (REAR+1) mod n == FRONT  
Full: (FRONT+1) mod n == REAR, empty: REAR == FRONT 
Discuss it
Question 7 Explanation:
Suppose we start filling the queue. Let the maxQueueSize ( Capacity of the Queue) is 4. So the size of the array which is used to implement this circular queue is 5, which is n. In the begining when the queue is empty, FRONT and REAR point to 0 index in the array. REAR represents insertion at the REAR index. FRONT represents deletion from the FRONT index. enqueue("a"); REAR = (REAR+1)%5; ( FRONT = 0, REAR = 1) enqueue("b"); REAR = (REAR+1)%5; ( FRONT = 0, REAR = 2) enqueue("c"); REAR = (REAR+1)%5; ( FRONT = 0, REAR = 3) enqueue("d"); REAR = (REAR+1)%5; ( FRONT = 0, REAR = 4) Now the queue size is 4 which is equal to the maxQueueSize. Hence overflow condition is reached. Now, we can check for the conditions. When Queue Full : ( REAR+1)%n = (4+1)%5 = 0 FRONT is also 0. Hence ( REAR + 1 ) %n is equal to FRONT. When Queue Empty : REAR was equal to FRONT when empty ( because in the starting before filling the queue FRONT = REAR = 0 ) Hence Option A is correct.
Question 8 
A PriorityQueue is implemented as a MaxHeap. Initially, it has 5 elements. The levelorder traversal of the heap is given below:
10, 8, 5, 3, 2
Two new elements ”1‘ and ”7‘ are inserted in the heap in that order. The levelorder traversal of the heap after the insertion of the elements is:
10, 8, 7, 5, 3, 2, 1  
10, 8, 7, 2, 3, 1, 5  
10, 8, 7, 1, 2, 3, 5  
10, 8, 7, 3, 2, 1, 5 
Discuss it
Question 8 Explanation:
See question 4 of http://www.geeksforgeeks.org/datastructuresandalgorithmsset22/
Question 9 
An implementation of a queue Q, using two stacks S1 and S2, is given below:
void insert(Q, x) { push (S1, x); } void delete(Q){ if(stackempty(S2)) then if(stackempty(S1)) then { print(“Q is empty”); return; } else while (!(stackempty(S1))){ x=pop(S1); push(S2,x); } x=pop(S2); }Let n insert and m (<=n) delete operations be performed in an arbitrary order on an empty queue Q. Let x and y be the number of push and pop operations performed respectively in the process. Which one of the following is true for all m and n?
n+m <= x < 2n and 2m <= y <= n+m  
n+m <= x < 2n and 2m<= y <= 2n  
2m <= x < 2n and 2m <= y <= n+m  
2m <= x <2n and 2m <= y <= 2n 
Discuss it
Question 9 Explanation:
The order in which insert and delete operations are performed matters here.
The best case: Insert and delete operations are performed alternatively. In every delete operation, 2 pop and 1 push operations are performed. So, total m+ n push (n push for insert() and m push for delete()) operations and 2m pop operations are performed.
The worst case: First n elements are inserted and then m elements are deleted. In first delete operation, n + 1 pop operations and n push operation are performed. Other than first, in all delete operations, 1 pop operation is performed. So, total m + n pop operations and 2n push operations are performed (n push for insert() and n push for delete())
Question 10 
Consider the following operation along with Enqueue and Dequeue operations on
queues, where k is a global parameter.
MultiDequeue(Q){ m = k while (Q is not empty and m > 0) { Dequeue(Q) m = m  1 } }What is the worst case time complexity of a sequence of n MultiDequeue() operations on an initially empty queue? (GATE CS 2013) (A) (B) (C) (D)
A  
B  
C  
D 
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Question 10 Explanation:
Since the queue is empty initially, the condition of while loop never becomes true. So the time complexity is [Tex]\Theta(n)[/Tex].
Question 11 
Consider the following pseudo code. Assume that IntQueue is an integer queue. What does the function fun do?
void fun(int n) { IntQueue q = new IntQueue(); q.enqueue(0); q.enqueue(1); for (int i = 0; i < n; i++) { int a = q.dequeue(); int b = q.dequeue(); q.enqueue(b); q.enqueue(a + b); ptint(a); } }
Prints numbers from 0 to n1  
Prints numbers from n1 to 0  
Prints first n Fibonacci numbers  
Prints first n Fibonacci numbers in reverse order. 
Discuss it
Question 11 Explanation:
The function prints first n Fibonacci Numbers. Note that 0 and 1 are initially there in q. In every iteration of loop sum of the two queue items is enqueued and the front item is dequeued.
Question 12 
Consider the following operation along with Enqueue and Dequeue operations on queues, where k is a global parameter.
MultiDequeue(Q){ m = k while (Q is not empty and m > 0) { Dequeue(Q) m = m  1 } }What is the worst case time complexity of a sequence of n MultiDequeue() operations on an initially empty queue? (GATE CS 2013)
(A) (B) (C) (D)
A  
B  
C  
D 
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Question 12 Explanation:
Question 13 
Suppose implementation supports an instruction REVERSE, which reverses the order of elements on the stack, in addition to the PUSH and POP instructions. Which one of the following statements is TRUE with respect to this modified stack?
A queue cannot be implemented using this stack.  
A queue can be implemented where ENQUEUE takes a single instruction and DEQUEUE takes a sequence of two instructions.
 
A queue can be implemented where ENQUEUE takes a sequence of three instructions and DEQUEUE takes a single instruction.
 
A queue can be implemented where both ENQUEUE and DEQUEUE take a single instruction each. 
Discuss it
Question 13 Explanation:
To DEQUEUE an item, simply POP.
To ENQUEUE an item, we can do following 3 operations
1) REVERSE
2) PUSH
3) REVERSE
Question 14 
A queue is implemented using an array such that ENQUEUE and DEQUEUE operations are performed efficiently. Which one of the following statements is CORRECT (n refers to the number of items in the queue)?
Both operations can be performed in O(1) time  
At most one operation can be performed in O(1) time but the worst case time for the other operation will be Ω(n)  
The worst case time complexity for both operations will be Ω(n)  
Worst case time complexity for both operations will be Ω(log n) 
Discuss it
Question 14 Explanation:
We can use circular array to implement both in O(1) time. See below article for details.
Question 15 
Let Q denote a queue containing sixteen numbers and S be an empty stack. Head(Q) returns the element at the head of the queue Q without removing it from Q. Similarly Top(S) returns the element at the top of S without removing it from S. Consider the algorithm given below.
The maximum possible number of iterations of the while loop in the algorithm is______
[This Question was originally a FillintheBlanks question]
16  
32  
256  
64 
Discuss it
Question 15 Explanation:
The worst case happens when the queue is sorted in decreasing order. In worst case, loop runs n*n times.
Queue: 4 3 2 1 Stack: Empty 3 2 1 4 3 2 1 4 Empty 2 1 4 3 2 1 4 3 Empty 1 4 3 2 1 4 3 2 Empty 4 3 2 1 3 2 1 4 3 2 4 1 2 4 1 3 2 4 3 1 4 3 1 2 3 1 2 4 3 4 1 2 4 1 2 3 Empty 1 2 3 4
Question 16 
Suppose you are given an implementation of a queue of integers. The operations that can be performed on the queue are:
i. isEmpty (Q) — returns true if the queue is empty, false otherwise.
ii. delete (Q) — deletes the element at the front of the queue and returns its value.
iii. insert (Q, i) — inserts the integer i at the rear of the queue.
Consider the following function:
i. isEmpty (Q) — returns true if the queue is empty, false otherwise.
ii. delete (Q) — deletes the element at the front of the queue and returns its value.
iii. insert (Q, i) — inserts the integer i at the rear of the queue.
Consider the following function:
void f (queue Q) { int i ; if (!isEmpty(Q)) { i = delete(Q); f(Q); insert(Q, i); } }What operation is performed by the above function f ?
Leaves the queue Q unchanged  
Reverses the order of the elements in the queue Q  
Deletes the element at the front of the queue Q and inserts it at the rear keeping the other elements in the same order  
Empties the queue Q

Discuss it
Question 16 Explanation:
As it is recursive call, and removing from front while inserting from end, that means last element will be deleted at last and will be inserted 1^{st} in the new queue. And like that it will continue till first call executes insert(Q,i) function.
So, the queue will be in reverse.
So, the queue will be in reverse.
There are 16 questions to complete.