Priority Queue Introduction and Concept in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a priority queue of integers in C++.

DSA C++

std::priority_queue<int> [1];

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

Apq

Bqueue

Cstack

Dlist

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated variable names like 'stack' or 'list' which are other data structures.

Forgetting to declare a variable name.

The standard name for the priority queue variable here is pq. This declares a priority queue of integers.

2fill in blank

medium

Complete the code to insert the integer 10 into the priority queue named pq.

DSA C++

pq.[1](10);

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

Apush

Binsert

Cadd

Denqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using insert which is not a method of priority_queue.

Using enqueue which is used in other queue types.

In C++, the method to add an element to a priority queue is push.

3fill in blank

hard

Fix the error in the code to get the top element from the priority queue named pq.

DSA C++

int topElement = pq.[1]();

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

Afirst

Btop

Cpeek

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using front() which is for other queue types.

Using peek() which is not a C++ priority queue method.

The correct method to access the highest priority element in a C++ priority queue is top().

4fill in blank

hard

Fill both blanks to remove the top element from the priority queue and then check if it is empty.

DSA C++

pq.[1]();
bool empty = pq.[2]();

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

Apop

Bpush

Cempty

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using size() to check emptiness instead of empty().

Use pop() to remove the top element and empty() to check if the priority queue is empty.

5fill in blank

hard

Fill all three blanks to create a min priority queue of integers using C++ STL.

DSA C++

std::priority_queue<int, std::vector<int>, [1]> [2];

[2].push(5);
[2].push(1);
[2].push(3);

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

Astd::greater<int>

BminPQ

Cpq

Dstd::less<int>

Attempts:

3 left

💡 Hint

Common Mistakes

Using std::less<int> which creates a max priority queue.

Using inconsistent variable names.

To create a min priority queue, use std::greater<int> as the comparator. The variable name is minPQ, used consistently.

Practice

1. What is the main property of a priority queue in data structures?

easy

A. Elements are always served in the order they were inserted

B. Elements are served based on their priority, not just insertion order

C. It stores elements in sorted order permanently

D. It only allows insertion of unique elements

Solution

1. Step 1: Understand priority queue behavior

   A priority queue serves elements based on priority, not just the order they arrive. Options A, C, and D describe other data structures or incorrect properties.

2. Final Answer:

   Elements are served based on their priority, not just insertion order -> Option B

3. Quick Check:

   Priority queue = priority-based serving [OK]

Hint: Remember: priority queue serves highest priority first [OK]

Common Mistakes:

• Confusing priority queue with regular queue
• Thinking elements are always sorted inside
• Assuming only unique elements allowed

2. Which of the following is the correct way to declare a max priority queue of integers in C++ STL?

easy

A. std::priority_queue<int> pq;

B. std::priority_queue pq;

C. std::priority_queue pq;

D. std::priority_queue pq;

Solution

1. Step 1: Recall default priority queue type

   By default, std::priority_queue is a max heap using std::vector and std::less. std::priority_queue<int> pq; uses default syntax. B creates min priority queue. C and D use invalid container type std::list.

2. Final Answer:

   std::priority_queue<int> pq; -> Option A

3. Quick Check:

   Default priority queue is max heap [OK]

Hint: Default priority_queue<int> is max heap in C++ STL [OK]

Common Mistakes:

• Using std::greater<int> for max heap (it's for min heap)
• Using unsupported container types like std::list
• Overcomplicating declaration when default works

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

std::priority_queue<int> pq;
pq.push(10);
pq.push(5);
pq.push(20);
std::cout << pq.top() << std::endl;
pq.pop();
std::cout << pq.top() << std::endl;

medium

A. 20\n10

B. 10\n5

C. 5\n10

D. 20\n5

Solution

1. Step 1: Insert elements into max priority queue

   Elements 10, 5, 20 pushed; max 20 at top, print 20. After pop, 20 removed, next max 10, print 10.

2. Final Answer:

   20\n10 -> Option A

3. Quick Check:

   Max heap top changes from 20 to 10 after pop [OK]

Hint: Top always shows current max in max priority queue [OK]

Common Mistakes:

• Confusing priority queue with FIFO queue
• Assuming pop removes smallest element
• Mixing up order of outputs

4. Identify the error in the following C++ code using priority queue:

std::priority_queue<int> pq;
pq.push(15);
pq.push(30);
pq.pop();
std::cout << pq.top() << std::endl;
pq.pop();
pq.pop();

medium

A. Missing semicolon after pq.push(30)

B. Using std::cout without including <iostream>

C. Calling pop() on empty priority queue causes runtime error

D. Priority queue cannot store integers

Solution

1. Step 1: Trace push and pop operations

   Push 15, 30; first pop removes 30, top now 15 printed; second pop removes 15; third pop on empty queue causes runtime error.

2. Final Answer:

   Calling pop() on empty priority queue causes runtime error -> Option C

3. Quick Check:

   Pop on empty queue = error [OK]

Hint: Never pop from empty priority queue [OK]

Common Mistakes:

• Ignoring empty check before pop
• Assuming pop returns value
• Confusing syntax errors with runtime errors

5. You want to maintain a priority queue that always gives the smallest element first (a min priority queue). Which of the following is the correct way to declare it in C++ STL?

hard

A. std::priority_queue pq;

B. std::priority_queue<int> pq;

C. std::priority_queue pq;

D. std::priority_queue pq;

Solution

1. Step 1: Understand default and min heap declaration

   Default std::priority_queue is max heap (std::less). For min heap (smallest first), use std::greater with std::vector. A and B are max heaps; D uses unsupported std::list.

2. Final Answer:

   std::priority_queue<int, std::vector<int>, std::greater<int>> pq; -> Option D

3. Quick Check:

   Min heap = priority_queue with std::greater [OK]

Hint: Use std::greater for min priority queue in C++ STL [OK]

Common Mistakes:

• Using std::less for min heap
• Using unsupported containers like std::list
• Forgetting to specify comparator