Enqueue Using Linked List in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node for enqueue operation.

DSA C

struct Node* newNode = (struct Node*)malloc(sizeof([1]));

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

AQueue

BNode

Cint

Dstruct

Attempts:

3 left

💡 Hint

Common Mistakes

Using sizeof(int) instead of sizeof(Node).

Using sizeof(struct) which is invalid.

We allocate memory for a new node, so we use sizeof(Node).

2fill in blank

medium

Complete the code to assign the data value to the new node.

DSA C

newNode->data = [1];

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Avalue

BNULL

CnewNode

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning newNode->data = data (undefined variable).

Assigning newNode->data = NULL.

We assign the input value to the data field of the new node.

3fill in blank

hard

Fix the error in linking the new node at the end of the queue.

DSA C

queue->rear->[1] = newNode;

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Adata

Bprev

Crear

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev instead of next.

Assigning to data or rear fields.

The next pointer of the rear node should point to the new node.

4fill in blank

hard

Fill both blanks to update the rear pointer and set the new node's next to NULL.

DSA C

queue->rear = [1];
newNode->[2] = NULL;

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AnewNode

Bqueue

Cnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Setting rear to queue instead of newNode.

Setting newNode->prev to NULL instead of next.

The rear pointer should point to the new node, and newNode's next should be NULL.

5fill in blank

hard

Fill all three blanks to handle the empty queue case and enqueue the new node correctly.

DSA C

if (queue->rear == NULL) {
    queue->front = [1];
    queue->rear = [2];
} else {
    queue->rear->next = [3];
}

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

AnewNode

Bqueue

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning rear or front to queue instead of newNode.

Assigning rear->next to NULL instead of newNode.

When the queue is empty, both front and rear point to newNode. Otherwise, link newNode after rear.

Practice

1. What does the enqueue operation do in a queue implemented using a linked list?

easy

A. Deletes the entire queue

B. Adds an element at the end (rear) of the queue

C. Checks if the queue is empty

D. Removes an element from the front of the queue

Solution

1. Step 1: Understand enqueue operation

   Enqueue means adding a new element to the queue.

2. Step 2: Identify position of insertion in linked list queue

   In a linked list queue, new elements are added at the rear (end) to maintain order.

3. Final Answer:

   Adds an element at the end (rear) of the queue -> Option B

4. Quick Check:

   Enqueue = Add at rear [OK]

Hint: Enqueue always adds at the rear in a queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue operation
• Thinking enqueue adds at the front
• Mixing queue with stack operations

2. Which of the following is the correct syntax to create a new node for enqueue in C linked list?

easy

A. newNode = malloc(sizeof(Node));

B. struct Node newNode = (struct Node*)malloc(sizeof(struct Node));

C. Node newNode = malloc(sizeof(Node));

D. struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));

Solution

1. Step 1: Allocate memory for new node

   In C, use malloc with sizeof(struct Node) to allocate memory.

2. Step 2: Correct pointer declaration and casting

   Declare pointer as struct Node* and cast malloc result to (struct Node*).

3. Final Answer:

   struct Node* newNode = (struct Node*)malloc(sizeof(struct Node)); -> Option D

4. Quick Check:

   Correct malloc syntax = struct Node* newNode = (struct Node*)malloc(sizeof(struct Node)); [OK]

Hint: Always cast malloc to (struct Node*) in C [OK]

Common Mistakes:

• Missing struct keyword in declaration
• Not casting malloc result
• Using wrong variable types

3. What will be the state of the queue after executing the following code?

struct Node {
  int data;
  struct Node* next;
};

struct Node* front = NULL;
struct Node* rear = NULL;

// Enqueue 10
struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
newNode->data = 10;
newNode->next = NULL;
if (rear == NULL) {
  front = rear = newNode;
} else {
  rear->next = newNode;
  rear = newNode;
}

// Enqueue 20
newNode = (struct Node*)malloc(sizeof(struct Node));
newNode->data = 20;
newNode->next = NULL;
rear->next = newNode;
rear = newNode;

medium

A. 10 -> NULL

B. 20 -> 10 -> NULL

C. 10 -> 20 -> NULL

D. 20 -> NULL

Solution

1. Step 1: Enqueue 10 when queue is empty

   Both front and rear point to new node with data 10.

2. Step 2: Enqueue 20 when queue has one element

   rear->next points to new node with data 20, rear moves to this node.

3. Final Answer:

   10 -> 20 -> NULL -> Option C

4. Quick Check:

   Order of enqueue = 10 then 20 [OK]

Hint: New nodes link at rear, order stays FIFO [OK]

Common Mistakes:

• Reversing order of nodes
• Not updating rear pointer
• Forgetting to set next to NULL

4. Identify the error in the following enqueue function for a linked list queue:

void enqueue(int value) {
  struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
  newNode->data = value;
  newNode->next = NULL;
  if (front == NULL) {
    front = newNode;
  } else {
    rear->next = newNode;
    rear = newNode;
  }
}

medium

A. rear pointer is not updated when queue is empty

B. newNode->next should point to front

C. malloc size is incorrect

D. front pointer is updated incorrectly

Solution

1. Step 1: Check rear pointer update when queue is empty

   When front is NULL, only front = newNode is executed, rear remains uninitialized.

2. Step 2: No rear update in if branch

   The else block sets rear = newNode, but it is skipped when queue is empty.

3. Final Answer:

   rear pointer is not updated when queue is empty -> Option A

4. Quick Check:

   rear must be set when queue is empty [OK]

Hint: Always update rear when queue was empty [OK]

Common Mistakes:

• Not setting rear when queue is empty
• Incorrect next pointer assignment
• Wrong malloc size or casting

5. Given a linked list queue with elements 5 -> 15 -> 25 -> NULL, what will be the queue after enqueueing 35 and then 45? Show the final linked list state.

hard

A. 5 -> 15 -> 25 -> 35 -> 45 -> NULL

B. 45 -> 35 -> 25 -> 15 -> 5 -> NULL

C. 5 -> 15 -> 25 -> NULL

D. 35 -> 45 -> 5 -> 15 -> 25 -> NULL

Solution

1. Step 1: Understand enqueue adds at rear

   Enqueue adds new nodes at the end of the queue to maintain order.

2. Step 2: Add 35 and then 45 at the rear

   After adding 35, queue becomes 5 -> 15 -> 25 -> 35 -> NULL. Then adding 45 results in 5 -> 15 -> 25 -> 35 -> 45 -> NULL.

3. Final Answer:

   5 -> 15 -> 25 -> 35 -> 45 -> NULL -> Option A

4. Quick Check:

   Enqueue order preserved at rear [OK]

Hint: Enqueue appends at rear, order stays same [OK]

Common Mistakes:

• Adding new nodes at front instead of rear
• Reversing the order of elements
• Not updating rear pointer after enqueue