Enqueue Using Linked List in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'enqueue' mean in the context of a queue data structure?

Enqueue means adding an element to the end (rear) of the queue.

beginner

In a linked list implementation of a queue, where is the new node added during enqueue?

The new node is added at the rear (end) of the linked list.

intermediate

Why do we keep a 'rear' pointer in a linked list queue?

To quickly add new nodes at the end without traversing the whole list.

beginner

What happens if the queue is empty when we enqueue a new element using a linked list?

Both 'front' and 'rear' pointers are set to the new node because it is the only element.

beginner

Show the basic steps to enqueue an element in a linked list queue.

1. Create a new node with the data.
2. Set new node's next to NULL.
3. If queue is empty, set front and rear to new node.
4. Else, set rear's next to new node and update rear to new node.

What pointer do we update when we enqueue a new element in a linked list queue?

A tail's previous

B front

C head

D rear

We update the 'rear' pointer to point to the newly added node at the end.

When the queue is empty, what do we do during enqueue?

A Set front and rear to the new node

B Only update rear pointer

C Only update front pointer

D Do nothing

Both front and rear pointers must point to the new node as it is the only element.

What is the time complexity of enqueue operation in a linked list queue?

A O(n)

B O(log n)

C O(1)

D O(n^2)

Enqueue is O(1) because we add the new node directly at the rear using the rear pointer.

Which of these is NOT a step in enqueue using linked list?

A Traverse entire list to find rear

B Update rear pointer

C Update rear's next pointer

D Create new node

We do NOT traverse the list because we keep a rear pointer for direct access.

What does the 'next' pointer of the new node point to after enqueue?

A previous node

B NULL

C front node

D itself

The new node is at the end, so its next pointer is NULL.

Explain how enqueue operation works in a linked list implementation of a queue.

Think about how to add at the end efficiently.

Why is it important to maintain both front and rear pointers in a linked list queue?

Consider how operations happen at both ends.

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