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Top MCQs on Queue Data Structure with Answers

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 popped 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

Question 2

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

Question 3

Which of the following operations on a queue data structure has a time complexity of O(1)? 

A) Enqueue 
B) Dequeue 
C) Peek 
D) Clear

  • A and B

  • B only

  • C only

  • A and D 

Question 4

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

Question 5

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

Question 6

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

Question 7

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);
      else while (!(stack-empty(S1))){

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

Question 8

Consider the following operation along with Enqueue and Dequeue operations on queues, where k is a global parameter.

m = k
while (Q is not empty and m > 0) {
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) [Tex]\\Theta(n)  [/Tex](B) [Tex]\\Theta(n + k)  [/Tex](C) [Tex]\\Theta(nk)  [/Tex](D) [Tex]\\Theta(n^2) [/Tex]

  • A

  • B

  • C

  • D

Question 9

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();
    for (int i = 0; i < n; i++)
        int a = q.dequeue();
        int b = q.dequeue();
        q.enqueue(a + b);
  • Prints numbers from 0 to n-1

  • Prints numbers from n-1 to 0

  • Prints first n Fibonacci numbers

  • Prints first n Fibonacci numbers in reverse order.

Question 10

Which of the following is NOT a common operation in a queue data structure? 

  • Enqueue 

  • Dequeue 

  • Peek 

  • Shuffle 

There are 30 questions to complete.

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