Why Enqueue Using Linked List in DSA C?

The Big Idea

What if you could add to a line instantly without moving anyone else?

The Scenario

Imagine you have a long line of people waiting to buy tickets. You try to add each new person by shifting everyone one step back manually. This takes a lot of time and effort as the line grows.

The Problem

Manually moving each person back every time a new person arrives is slow and tiring. It's easy to make mistakes, like losing track of someone or mixing the order. This gets worse as the line gets longer.

The Solution

Using a linked list to add people to the end of the line lets you add new arrivals quickly without moving anyone else. You just connect the new person to the end, keeping the order perfect and saving time.

Before vs After

✗ Before

for (int i = size; i > 0; i--) {
  queue[i] = queue[i-1];
}
queue[0] = new_person;

✓ After

new_node->next = NULL;
if (rear == NULL) {
  front = rear = new_node;
} else {
  rear->next = new_node;
  rear = new_node;
}

What It Enables

This lets you add items to a queue quickly and safely, no matter how long it grows.

Real Life Example

Think of a printer queue where documents arrive one after another. Using a linked list to enqueue documents means the printer always knows the correct order without delays.

Key Takeaways

Manually shifting elements is slow and error-prone.

Linked list enqueue adds new items at the end efficiently.

Maintains order and works well for growing queues.

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