Enqueue Operation in DSA C++ - Execution Trace

Concept Flow - Enqueue Operation

Create new node with value

↓

Is queue empty?

Yes→Set head and tail to new node

|No↓

Set tail->next to new node

↓

Update tail to new node

↓

Done

This flow shows how a new node is added at the end of the queue. If empty, head and tail point to it. Otherwise, tail's next points to new node, then tail updates.

Execution Sample

DSA C++

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

void enqueue(Node*& head, Node*& tail, int val) {
  Node* newNode = new Node{val, nullptr};
  if (!head) head = tail = newNode;
  else { tail->next = newNode; tail = newNode; }
}

This code adds a new value to the end of a linked list queue, updating head and tail pointers accordingly.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with value 10	None	newNode -> Node(10, nullptr)	null
2	Check if queue is empty (head == nullptr)	None	head=nullptr, tail=nullptr	null
3	Queue empty: set head and tail to newNode	Node(10)	head -> Node(10), tail -> Node(10)	head -> [10] -> null tail -> [10] -> null
4	Create new node with value 20	Node(10)	newNode -> Node(20, nullptr)	head -> [10] -> null tail -> [10] -> null
5	Queue not empty: set tail->next to newNode	Node(10) -> Node(20)	tail->next = newNode	head -> [10] -> [20] -> null tail -> [10] -> [20] -> null
6	Update tail to newNode	Node(10) -> Node(20)	tail = newNode	head -> [10] -> [20] -> null tail -> [20] -> null
7	Create new node with value 30	Node(10) -> Node(20)	newNode -> Node(30, nullptr)	head -> [10] -> [20] -> null tail -> [20] -> null
8	Queue not empty: set tail->next to newNode	Node(10) -> Node(20) -> Node(30)	tail->next = newNode	head -> [10] -> [20] -> [30] -> null tail -> [20] -> [30] -> null
9	Update tail to newNode	Node(10) -> Node(20) -> Node(30)	tail = newNode	head -> [10] -> [20] -> [30] -> null tail -> [30] -> null
10	End of enqueue operations	Node(10) -> Node(20) -> Node(30)	head -> Node(10), tail -> Node(30)	head -> [10] -> [20] -> [30] -> null tail -> [30] -> null

💡 No more values to enqueue; queue updated with all nodes.

Variable Tracker

Variable	Start	After Step 3	After Step 6	After Step 9	Final
head	nullptr	Node(10)	Node(10)	Node(10)	Node(10)
tail	nullptr	Node(10)	Node(20)	Node(30)	Node(30)
newNode	nullptr	Node(10)	Node(20)	Node(30)	nullptr (end)

Key Moments - 3 Insights

Why do we set both head and tail to the new node when the queue is empty?

Because when the queue is empty, there is no first or last node. The new node is both the start (head) and end (tail) of the queue, as shown in step 3 of the execution_table.

Why do we update tail->next before moving tail to the new node?

We must link the current last node's next pointer to the new node first, so the chain stays connected. Then we update tail to point to the new node, as seen in steps 5 and 6.

What happens if we forget to update tail after adding a new node?

The tail pointer would still point to the old last node, so future enqueues would break the chain or add nodes incorrectly. The execution_table shows tail updating at steps 6 and 9 to avoid this.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what do head and tail point to?

A head points to null, tail points to new node

B head points to new node, tail points to null

C Both point to the new node with value 10

D Both point to null

💡 Hint

Check the 'Pointer Changes' and 'Visual State' columns at step 3.

At which step does the tail pointer first move to a new node after the initial enqueue?

A Step 5

B Step 6

C Step 4

D Step 7

💡 Hint

Look at the 'Pointer Changes' column to see when tail is assigned a new node.

If we enqueue a new node with value 40 after step 9, what will tail point to after the operation?

A Node with value 40

B Node with value 20

C Node with value 30

D Head node

💡 Hint

Refer to the pattern of tail updates in steps 6 and 9 in the execution_table.

Concept Snapshot

Enqueue Operation in Queue:
- Create new node with value and next=nullptr
- If queue empty: head=tail=new node
- Else: tail->next=new node, tail=new node
- Adds element at queue's end
- Maintains FIFO order
- Updates pointers carefully to keep chain intact

Full Transcript

The enqueue operation adds a new element to the end of a queue implemented as a linked list. First, a new node is created with the given value and its next pointer set to null. If the queue is empty (head is null), both head and tail pointers are set to this new node, making it the only element. If the queue is not empty, the current tail's next pointer is set to the new node, and then the tail pointer itself is updated to point to the new node. This keeps the queue connected and maintains the order of elements. The execution table shows step-by-step how nodes are created and linked, and how head and tail pointers change. Key moments clarify why both head and tail are set when empty, why tail->next is updated before tail, and the importance of updating tail to avoid breaking the queue. The visual quiz tests understanding of pointer changes and queue state after each enqueue step.

Practice

1. What does the enqueue operation do in a queue data structure?

easy

A. Returns the size of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Adds an element to the end of the queue

Solution

1. Step 1: Understand the queue structure

   A queue works like a line where elements are added at the back and removed from the front.

2. Step 2: Define the enqueue operation

   Enqueue means adding a new element to the end of the queue to wait its turn.

3. Final Answer:

   Adds an element to the end of the queue -> Option D

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue always adds at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue (removal)
• Thinking enqueue removes elements
• Mixing enqueue with checking queue size

2. Which of the following is the correct syntax to enqueue an element 5 into a queue named q in C++ STL?

easy

A. q.push(5);

B. q.add(5);

C. q.enqueue(5);

D. q.insert(5);

Solution

1. Step 1: Recall C++ STL queue methods

   The standard queue in C++ uses push() to add elements at the back.

2. Step 2: Match the correct method

   enqueue is not a method in STL queue; push is used instead.

3. Final Answer:

   q.push(5); -> Option A

4. Quick Check:

   STL queue uses push() to enqueue [OK]

Hint: Use push() to add elements in C++ STL queue [OK]

Common Mistakes:

• Using enqueue() which is not in STL queue
• Using add() or insert() which are not queue methods
• Confusing push() with pop()

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

std::queue<int> q;
q.push(10);
q.push(20);
q.push(30);
q.pop();
std::cout << q.front();

medium

A. 10

B. Compilation error

C. 20

D. 30

Solution

1. Step 1: Trace the enqueue operations

   Elements 10, 20, 30 are added in order: queue = [10, 20, 30]

2. Step 2: Apply the dequeue operation

   pop() removes the front element 10, so queue = [20, 30]

3. Step 3: Check the front element

   front() now returns 20, the new front.

4. Final Answer:

   20 -> Option C

5. Quick Check:

   After pop, front = 20 [OK]

Hint: pop removes front; front() shows new front [OK]

Common Mistakes:

• Assuming front remains 10 after pop
• Confusing pop with removing from back
• Expecting output 30 instead of 20

4. Identify the error in the following code that tries to enqueue an element into a queue:

std::queue<int> q;
q.enqueue(15);

medium

A. Missing #include <queue>

B. enqueue is not a member function of std::queue

C. Queue must be initialized with size

D. Cannot enqueue integer values

Solution

1. Step 1: Check std::queue methods

   std::queue does not have an enqueue method; it uses push to add elements.

2. Step 2: Identify the error cause

   Calling enqueue causes a compilation error because the method does not exist.

3. Final Answer:

   enqueue is not a member function of std::queue -> Option B

4. Quick Check:

   Use push() not enqueue() in std::queue [OK]

Hint: std::queue uses push(), not enqueue() [OK]

Common Mistakes:

• Assuming enqueue() exists in std::queue
• Thinking queue needs size at initialization
• Believing integers cannot be enqueued

5. You have a queue initially empty. You perform these operations:

enqueue(1)
enqueue(2)
dequeue()
enqueue(3)
dequeue()
enqueue(4)

What is the final state of the queue (front to back)?

hard

A. 3 -> 4 -> null

B. 2 -> 3 -> 4 -> null

C. 1 -> 3 -> 4 -> null

D. 4 -> null

Solution

1. Step 1: Perform operations step-by-step

   Start empty: []
   enqueue(1): [1]
   enqueue(2): [1, 2]
   dequeue(): removes 1 -> [2]
   enqueue(3): [2, 3]
   dequeue(): removes 2 -> [3]
   enqueue(4): [3, 4]

2. Step 2: Write final queue state

   Queue front is 3, then 4, then null (end).

3. Final Answer:

   3 -> 4 -> null -> Option A

4. Quick Check:

   After operations, queue = 3,4 [OK]

Hint: Track each enqueue/dequeue step carefully [OK]

Common Mistakes:

• Forgetting to remove elements on dequeue
• Mixing order of elements after operations
• Assuming dequeue removes from back