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 node structure with an integer priority.

DSA C

typedef struct Node {
    int data;
    int [1];
    struct Node* next;
} Node;

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

Apriority

Bvalue

Cindex

Dweight

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic name like 'value' instead of 'priority'.

Confusing priority with data.

The field 'priority' stores the priority value for each node in the priority queue.

2fill in blank

medium

Complete the code to insert a new node into the priority queue based on priority order (higher priority first).

DSA C

void insert(Node** head, int data, int priority) {
    Node* newNode = malloc(sizeof(Node));
    newNode->data = data;
    newNode->priority = priority;
    newNode->next = NULL;

    if (*head == NULL || (*head)->priority < [1]) {
        newNode->next = *head;
        *head = newNode;
    } else {
        Node* temp = *head;
        while (temp->next != NULL && temp->next->priority >= priority) {
            temp = temp->next;
        }
        newNode->next = temp->next;
        temp->next = newNode;
    }
}

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

AnewNode->priority

Bdata

CnewNode->data

Dpriority

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with 'data' instead of 'priority'.

Using 'priority' instead of 'newNode->priority' in the condition.

We compare the priority of the new node with the head node's priority to decide insertion at the front.

3fill in blank

hard

Fix the error in the dequeue function that removes the highest priority node from the queue.

DSA C

int dequeue(Node** head) {
    if (*head == NULL) {
        return -1; // Queue empty
    }
    Node* temp = *head;
    int data = temp->data;
    *head = [1];
    free(temp);
    return data;
}

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

A*head->next

Btemp->next

Chead->next

D(*head)->next

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'head->next' which is invalid because head is a double pointer.

Using '*head->next' which is parsed incorrectly.

To remove the head node, we set *head to the next node using (*head)->next.

4fill in blank

hard

Fill both blanks to complete the function that prints the priority queue nodes in order.

DSA C

void printQueue(Node* head) {
    Node* current = head;
    while (current != NULL) {
        printf("Data: %d, Priority: %d\n", current->[1], current->[2]);
        current = current->next;
    }
}

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

Adata

Bnext

Cpriority

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' or 'value' instead of 'data' or 'priority'.

Mixing up the order of fields.

We print the 'data' and 'priority' fields of each node.

5fill in blank

hard

Fill all three blanks to complete the function that checks if the priority queue is empty.

DSA C

int isEmpty(Node* [1]) {
    return [2] == [3];
}

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

Ahead

CNULL

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Using an undefined variable like 'temp'.

Comparing to 0 instead of NULL.

We check if the pointer 'head' is equal to NULL to determine if the queue is empty.

Practice

1. What is the main characteristic of a priority queue compared to a regular queue?

easy

A. It does not allow insertion of new elements after creation

B. Elements are always dequeued in the order they were enqueued

C. It only stores elements of the same value

D. Elements are dequeued based on their priority, not just arrival order

Solution

1. Step 1: Understand queue behavior

   A regular queue removes elements in the order they arrive (FIFO).

2. Step 2: Understand priority queue behavior

   A priority queue removes elements based on priority, not arrival order.

3. Final Answer:

   Elements are dequeued based on their priority, not just arrival order -> Option D

4. Quick Check:

   Priority queue = priority-based removal [OK]

Hint: Priority queue removes highest priority first, not first-in [OK]

Common Mistakes:

• Thinking priority queue is same as FIFO queue
• Assuming elements must be unique
• Believing insertion order is always preserved

2. Which of the following is the correct way to declare a priority queue in C using the standard library?

easy

A. There is no built-in priority queue in standard C

B. struct priority_queue pq;

C. PriorityQueue pq = new PriorityQueue();

D. priority_queue<int> pq;

Solution

1. Step 1: Recall C standard library features

   The C standard library does not include a built-in priority queue type.

2. Step 2: Analyze options

   The other options show declaration syntax resembling C++ (priority_queue or struct) or Java (PriorityQueue class), none of which are standard in C.

3. Final Answer:

   There is no built-in priority queue in standard C -> Option A

4. Quick Check:

   C standard library lacks priority queue [OK]

Hint: C standard library lacks priority queue type [OK]

Common Mistakes:

• Confusing C with C++ STL syntax
• Assuming priority queue is built-in in C
• Using Java or C++ syntax in C code

3. Consider the following pseudocode for a max-priority queue operations:

insert(5)
insert(3)
insert(10)
extract_max()
insert(1)
extract_max()

What is the sequence of extracted elements?

medium

A. [10, 3]

B. [5, 3]

C. [10, 5]

D. [3, 1]

Solution

1. Step 1: Insert elements into max-priority queue

   Insert 5, 3, 10. The max element is 10.

2. Step 2: Extract max elements in order

   First extract_max() returns 10. Then insert 1. Next extract_max() returns 5 (next highest).

3. Final Answer:

   [10, 5] -> Option C

4. Quick Check:

   Extracted max elements = [10, 5] [OK]

Hint: Extract max returns highest inserted so far [OK]

Common Mistakes:

• Extracting elements in insertion order
• Confusing min and max priority
• Ignoring new insertions before extraction

4. The following C-like pseudocode tries to implement a priority queue insertion but has a bug:

void insert(int pq[], int *size, int value) {
  pq[*size] = value;
  (*size)++;
  int i = *size - 1;
  while (i > 0 && pq[i] < pq[(i - 1) / 2]) {
    int temp = pq[i];
    pq[i] = pq[(i - 1) / 2];
    pq[(i - 1) / 2] = temp;
    i = (i - 1) / 2;
  }
}

What is the bug in this insertion logic?

medium

A. It uses < instead of > for max-heap property

B. It increments size before inserting value

C. It swaps with wrong parent index

D. It does not update the heap after insertion

Solution

1. Step 1: Understand heap property for max-heap

   Max-heap requires parent >= children, so swap if child > parent.

2. Step 2: Analyze condition in while loop

   Condition uses pq[i] < pq[parent], which is correct for min-heap, not max-heap.

3. Final Answer:

   It uses < instead of > for max-heap property -> Option A

4. Quick Check:

   Max-heap needs child > parent to swap [OK]

Hint: Max-heap swaps when child > parent, not < [OK]

Common Mistakes:

• Using min-heap condition for max-heap
• Incorrect parent index calculation
• Not incrementing size correctly

5. You have a priority queue that stores tasks with integer priorities. You want to process tasks in order of highest priority but also keep track of the order tasks with the same priority were added. Which data structure modification best supports this?

hard

A. Use a simple array sorted by priority only

B. Use a max-heap with a timestamp to break ties

C. Use a queue ignoring priorities

D. Use a stack to process tasks in reverse order

Solution

1. Step 1: Understand requirement for priority and insertion order

   We want highest priority first, but for equal priority, process by insertion order.

2. Step 2: Choose data structure supporting priority and tie-break

   A max-heap with timestamps can break ties by earlier insertion time, preserving order among equals.

3. Final Answer:

   Use a max-heap with a timestamp to break ties -> Option B

4. Quick Check:

   Max-heap + timestamp = priority + insertion order [OK]

Hint: Add timestamp to priority for tie-breaking [OK]

Common Mistakes:

• Ignoring insertion order for equal priorities
• Using simple array without tie-break
• Using stack or queue ignoring priority