Queue vs Stack When to Use Which in DSA C++ - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack.

DSA C++

std::stack<int> s;
s.push([1]);

Drag options to blanks, or click blank then click option'

A10

Bfront()

Cpop()

Dempty()

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() inside push() instead of a value.

Trying to use front() which is for queues.

Using empty() which returns a bool, not a value.

To add an element to a stack, use push() with the value to add, like 10.

2fill in blank

medium

Complete the code to get the front element of the queue.

DSA C++

std::queue<int> q;
int frontElement = q.[1]();

Drag options to blanks, or click blank then click option'

Apop

Bpush

Ctop

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which removes the element instead of accessing it.

Using top() which is a stack method.

Using push() which adds elements.

To access the front element of a queue, use front() method.

3fill in blank

hard

Fix the error in the code to correctly remove the top element from the stack.

DSA C++

std::stack<int> s;
s.push(5);
s.[1]();

Drag options to blanks, or click blank then click option'

Afront

Bremove

Cpop

Ddequeue

Attempts:

3 left

💡 Hint

Common Mistakes

Using front() which is a queue method.

Using remove() which does not exist for stack.

Using dequeue() which is for queues.

To remove the top element from a stack, use pop() method.

4fill in blank

hard

Fill both blanks to create a queue and add an element to it.

DSA C++

std::queue<int> [1];
[2].push(20);

Drag options to blanks, or click blank then click option'

Aq

Bs

Cstack

Dqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in declaration and push.

Using s or stack which are usually for stacks, not queues.

Using queue as variable name which is allowed but inconsistent here.

We declare a queue named q and then use q.push(20) to add element 20.

5fill in blank

hard

Fill all three blanks to create a stack, push an element, and get the top element.

DSA C++

std::stack<int> [1];
[1].[2](15);
int topElement = [1].top();

Drag options to blanks, or click blank then click option'

Ast

Bpush

Cpop

Denqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of push() to add elements.

Using enqueue() which is not a stack method.

Using different variable names inconsistently.

We create a stack named st, push 15 onto it, then get the top element with top().

Practice

1. Which data structure should you use when you need to process elements in the exact order they arrive, like customers waiting in line?

easy

A. Graph

B. Stack

C. Tree

D. Queue

Solution

1. Step 1: Understand the order requirement

   Customers waiting in line are served in the order they arrive, which is First-In-First-Out (FIFO).

2. Step 2: Match data structure behavior

   A queue follows FIFO, so it fits this scenario perfectly.

3. Final Answer:

   Queue -> Option D

4. Quick Check:

   FIFO = Queue [OK]

Hint: Use queue for FIFO order tasks like waiting lines [OK]

Common Mistakes:

• Confusing stack (LIFO) with queue (FIFO)
• Choosing tree or graph which are not linear
• Ignoring order of processing

2. Which of the following is the correct way to push an element '5' onto a stack named myStack in C++ STL?

easy

A. myStack.push(5);

B. myStack.enqueue(5);

C. myStack.insert(5);

D. myStack.add(5);

Solution

1. Step 1: Recall stack insertion method in C++ STL

   The standard method to add an element to a stack is push().

2. Step 2: Verify the syntax

   Using myStack.push(5); correctly adds 5 to the top of the stack.

3. Final Answer:

   myStack.push(5); -> Option A

4. Quick Check:

   Stack insertion = push() [OK]

Hint: Use push() to add elements to a stack in C++ [OK]

Common Mistakes:

• Using enqueue which is for queues
• Using insert or add which are not stack methods
• Confusing stack methods with vector methods

3. What will be the output of the following C++ code snippet?

#include <iostream>
#include <stack>
using namespace std;

int main() {
    stack<int> s;
    s.push(10);
    s.push(20);
    s.push(30);
    cout << s.top() << endl;
    s.pop();
    cout << s.top() << endl;
    return 0;
}

medium

A. 30\n20

B. 10\n20

C. 20\n10

D. 30\n10

Solution

1. Step 1: Trace stack operations

   Elements pushed in order: 10 (bottom), 20, 30 (top). First cout prints top: 30.

2. Step 2: Pop and print new top

   Pop removes 30, new top is 20, second cout prints 20.

3. Final Answer:

   30\n20 -> Option A

4. Quick Check:

   Stack top after push and pop = 30 then 20 [OK]

Hint: Stack top shows last pushed element, pop removes it [OK]

Common Mistakes:

• Assuming queue order instead of stack
• Printing bottom element instead of top
• Confusing pop with peek

4. The following code is intended to implement a queue using C++ STL. Identify the error:

#include <iostream>
#include <queue>
using namespace std;

int main() {
    queue<int> q;
    q.push(1);
    q.push(2);
    cout << q.top() << endl;
    return 0;
}

medium

A. queue requires pop() before push()

B. queue cannot push elements

C. queue does not have a top() method

D. queue must be declared as stack

Solution

1. Step 1: Check queue methods

   Queue uses front() to access the first element, not top().

2. Step 2: Identify the error in code

   Using q.top() causes a compile error because top() is a stack method.

3. Final Answer:

   queue does not have a top() method -> Option C

4. Quick Check:

   Queue front() not top() [OK]

Hint: Use front() for queue, top() for stack [OK]

Common Mistakes:

• Using top() with queue
• Thinking queue needs pop before push
• Confusing queue and stack declarations

5. You want to reverse the order of words in a sentence efficiently. Which data structure is best suited for this task and why?

hard

A. Queue, because it preserves original order

B. Stack, because it reverses order using LIFO

C. Queue, because it supports random access

D. Stack, because it sorts elements automatically

Solution

1. Step 1: Understand the problem requirement

   Reversing word order means last word should come first, which is Last-In-First-Out (LIFO).

2. Step 2: Choose data structure matching LIFO

   Stack follows LIFO, so pushing words then popping reverses their order efficiently.

3. Final Answer:

   Stack, because it reverses order using LIFO -> Option B

4. Quick Check:

   Reverse order = Stack (LIFO) [OK]

Hint: Use stack to reverse order with LIFO behavior [OK]

Common Mistakes:

• Choosing queue which preserves order
• Thinking stack sorts elements automatically
• Confusing random access with reversal