# Queue

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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 |

**Queue**

**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 |

**Queue**

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Question 2 Explanation:

See http://www.geeksforgeeks.org/applications-of-queue-data-structure/ 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 |

**Queue**

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Question 3 Explanation:

A queue can be implemented using two stacks. See following for implementation.
http://www.geeksforgeeks.org/queue-using-stacks/

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 |

**Queue**

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Question 4 Explanation:

A stack can be implemented using two queues. Please see following for details.
http://www.geeksforgeeks.org/implement-stack-using-queue/

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 |

**Queue**

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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 |

**Queue**

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Question 6 Explanation:

To keep the

**F**irst**I**n**F**irst**O**ut 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 |

**Queue**

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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 beginning 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 Priority-Queue is implemented as a Max-Heap. Initially, it has 5 elements. The level-order 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 level-order 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 |

**Queue**

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Question 8 Explanation:

See question 4 of http://www.geeksforgeeks.org/data-structures-and-algorithms-set-22/

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(stack-empty(S2)) then if(stack-empty(S1)) then { print(“Q is empty”); return; } else while (!(stack-empty(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 |

**Queue**

**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 |

**Queue**

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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 .

There are 22 questions to complete.