Stacks, Queues and Deques Practice Questions for GATE DA: 5+ Solved Questions with Step-by-Step Solutions

    Solve 5+ Stacks, Queues and Deques practice questions for GATE DA with answers and detailed solutions. Free sample questions below.

    Chapter Roadmap: Stacks, Queues, and Deques

    Chapter Journey: Linear Data Structures

    01
    Stack Operations and Simulation
    Master Push, Pop, and Top. Learn to trace complex pseudocode executions manually.
    Weightage Hint: High frequency in simulation questions.
    02
    Queue and Deque Operations
    Understand F I F O behavior, circular queues, and double-ended insertion and removal.
    Weightage Hint: Essential for scheduling and buffering concepts.
    03
    ADT Properties: FIFO, LIFO, Lookup
    Formalize the abstract definitions. Match properties to real-world applications.
    Weightage Hint: Conceptual clarity for matching and theory questions.

    The Stack Intuition: The Plate Pile

    What is a Stack?

    A Stack is a linear data structure that follows a particular order in which operations are performed. The order may be LIFO (Last In First Out) or FILO (First In Last Out).

    Real-World Analogy

    Imagine a stack of books on a table:

    1. You place Book A down.
    2. You place Book B on top of A.
    3. You place Book C on top of B.

    To access Book A, you must first remove Book C, then Book B. The last item added (C) is the first one removed.

    Book C
    Book B
    Book A

    Key Terminology

    • Top: The only end where insertion and deletion occur.
    • Push: The operation of adding an element to the top.
    • Pop: The operation of removing the element from the top.

    Stacks, Queues and Deques: Solved Questions with Step-by-Step Explanations (5 Problems)

    Question 1 · Programming, Data Structures and Algorithms MCQ

    In a Stack data structure, the <code>Push</code> operation inserts a new element at which position?

    1. A.

      At the top of the stack

    2. B.

      At the bottom of the stack

    3. C.

      At the middle of the stack

    4. D.

      At a random position

    Correct Answer:

    A

    Step-by-Step Solution

    Key idea: This is a definition recall question about the Push operation, recognizable because it asks where a new element is placed in a stack. Step 1: Recall that a stack follows the LIFO (Last In, First Out) principle. Step 2: To maintain LIFO order, new elements must be added at the same end from which they will be removed. This end is called the top. Step 3: Therefore, Push always inserts at the top of the stack. Answer: Option A
    Question 2 · Programming, Data Structures and Algorithms MCQ

    Which stack operation removes and returns the topmost element?

    1. A.

      Push

    2. B.

      Peek

    3. C.

      Pop

    4. D.

      IsEmpty

    Correct Answer:

    C

    Step-by-Step Solution

    Key idea: This is a definition recall question about stack operations, recognizable because it asks which operation both removes and returns the top element. Step 1: Review the four standard stack operations. - Push: Adds an element to the top. Does not remove. - Peek (or Top): Returns the top element without removing it. - Pop: Removes the top element and returns it. - IsEmpty: Checks whether the stack is empty. Returns a boolean. Step 2: The question asks for the operation that both removes and returns. Only Pop does both. Answer: Option C
    Question 3 · Programming, Data Structures and Algorithms MCQ

    The access principle followed by a Stack data structure is called LIFO. What does LIFO stand for?

    1. A.

      Last In, Fast Out

    2. B.

      Last In, First Out

    3. C.

      Linear In, First Out

    4. D.

      Least In, First Out

    Correct Answer:

    B

    Step-by-Step Solution

    Key idea: This is a definition recall question, recognizable because it directly asks for the expansion of the acronym LIFO associated with stacks.

    Step 1: Recall that a Stack follows the Last In, First Out principle. The most recently added element is the first one to be removed.

    Step 2: Match this to the options. "Last In, First Out" is the correct expansion.

    Answer: Option B

    Question 4 · Programming, Data Structures and Algorithms MCQ

    A stack contains the elements from bottom to top. Which element(s) can be directly accessed without removing any other element?

    1. A.

      10, 20, and 30

    2. B.

      10 only

    3. C.

      20 only

    4. D.

      30 only

    Correct Answer:

    D

    Step-by-Step Solution

    Key idea: This is a conceptual question about stack access restrictions, recognizable because it asks which elements are directly accessible in a stack. Step 1: Recall the LIFO principle. A stack restricts all access to one end: the top. Step 2: In the given stack , the element 30 is at the top. Step 3: To access 20, you must first Pop 30. To access 10, you must first Pop 30 and 20. Step 4: Therefore, only the top element (30) can be directly accessed without removing anything else. Answer: Option D
    Question 5 · Programming, Data Structures and Algorithms MCQ

    An empty stack undergoes the following operations in order: <code>Push(5)</code>, <code>Push(9)</code>, <code>Pop()</code>. What is the value of the top element after these operations?

    1. A.

      9

    2. B.

      5

    3. C.

      14

    4. D.

      The stack is empty

    Correct Answer:

    B

    Step-by-Step Solution

    Key idea: This is a basic stack trace question, recognizable because it gives a short sequence of Push and Pop operations and asks for the resulting top element.

    Step 1: Start with an empty stack: .

    Step 2: <code>Push(5)</code>: Add 5 to the top. . Top is 5.

    Step 3: <code>Push(9)</code>: Add 9 to the top. . Top is 9.

    Step 4: <code>Pop()</code>: Remove the top element (9). . Top is now 5.

    Step 5: The top element after all operations is 5.

    Answer: Option B

    More practice questions in this unit

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    Stacks, Queues and Deques Practice Questions for GATE DA: 5+ Solved Questions with Step-by-Step Solutions

    Solve 5+ Stacks, Queues and Deques practice questions for GATE DA with answers and detailed solutions. Free sample questions below.

    A question from this chapter

    Question 1

    In a Stack data structure, the <code>Push</code> operation inserts a new element at which position?

    Question 2

    Which stack operation removes and returns the topmost element?

    Question 3

    The access principle followed by a Stack data structure is called LIFO. What does LIFO stand for?

    Question 4

    A stack contains the elements from bottom to top. Which element(s) can be directly accessed without removing any other element?

    Question 5

    An empty stack undergoes the following operations in order: <code>Push(5)</code>, <code>Push(9)</code>, <code>Pop()</code>. What is the value of the top element after these operations?

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