Enqueue Operation in DSA Python

Mental Model

Add a new item to the end of a line so it waits its turn.

Analogy: Like people lining up at a ticket counter, new people join at the back of the line.

front -> 1 -> 2 -> 3 -> null
rear ↑

Dry Run Walkthrough

Input: queue: 1 -> 2 -> 3 -> null, enqueue value 4

Goal: Add value 4 to the end of the queue

Step 1: Create new node with value 4

new_node = 4 -> null

Why: We need a new node to add to the queue

Step 2: Link current rear node (3) to new node (4)

1 -> 2 -> 3 -> [rear->4] -> null

Why: Attach new node at the end of the queue

Step 3: Update rear pointer to new node (4)

front -> 1 -> 2 -> 3 -> 4 -> null
rear ↑

Why: Rear must always point to the last node

Result:

front -> 1 -> 2 -> 3 -> 4 -> null
rear ↑

Annotated Code

DSA Python

class Node:
    def __init__(self, value):
        self.value = value
        self.next = None

class Queue:
    def __init__(self):
        self.front = None
        self.rear = None

    def enqueue(self, value):
        new_node = Node(value)  # create new node
        if self.rear is None:  # empty queue
            self.front = new_node
            self.rear = new_node
        else:
            self.rear.next = new_node  # link new node at end
            self.rear = new_node       # update rear pointer

    def __str__(self):
        result = []
        current = self.front
        while current:
            result.append(str(current.value))
            current = current.next
        return ' -> '.join(result) + ' -> null'

# Driver code
queue = Queue()
queue.enqueue(1)
queue.enqueue(2)
queue.enqueue(3)
print(queue)  # before enqueue
queue.enqueue(4)
print(queue)  # after enqueue

new_node = Node(value) # create new node

create a new node to add at the end

if self.rear is None: # empty queue

check if queue is empty to initialize front and rear

self.rear.next = new_node # link new node at end

attach new node after current rear

self.rear = new_node # update rear pointer

move rear pointer to new last node

Output

1 -> 2 -> 3 -> null

1 -> 2 -> 3 -> 4 -> null

Complexity Analysis

Time: O(1) because we add the new node directly at the rear without traversal

Space: O(1) because only one new node is created regardless of queue size

vs Alternative: Compared to adding at front or traversing to end (O(n)), enqueue at rear is faster and constant time

Edge Cases

empty queue

front and rear both point to the new node

DSA Python

if self.rear is None:  # empty queue

When to Use This Pattern

When you see a problem needing to add items in order and process them FIFO, reach for enqueue operation to add at the rear efficiently.

Common Mistakes

Mistake: Not updating the rear pointer after adding the new node
Fix: Always set rear = new_node after linking it

Mistake: Not handling empty queue case, causing front to remain None
Fix: Check if rear is None and set front and rear to new_node

Summary

Adds a new element to the end of the queue.

Use when you want to keep order and add items to the back of a waiting line.

Remember to update the rear pointer and handle empty queue separately.

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 new elements join at the back.

2. Step 2: Define enqueue operation

   Enqueue means adding an element to the end of this line.

3. Final Answer:

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

4. Quick Check:

   Enqueue = Add to end [OK]

Hint: Enqueue means add at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct syntax to enqueue an element 5 into a queue named q implemented as a Python list?

easy

A. q.enqueue(5)

B. q.push(5)

C. q.append(5)

D. q.insert(0, 5)

Solution

1. Step 1: Identify Python list method for adding at end

   Python lists use append() to add elements at the end.

2. Step 2: Match enqueue operation to list method

   Enqueue adds to the back, so q.append(5) is correct.

3. Final Answer:

   q.append(5) -> Option C

4. Quick Check:

   Enqueue in list = append() [OK]

Hint: Use append() to add at the end of a Python list [OK]

Common Mistakes:

• Using push() which is not a Python list method
• Using insert(0, 5) which adds at front
• Assuming enqueue is a built-in list method

3. What will be the output of the following code?

queue = []
queue.append(10)
queue.append(20)
queue.append(30)
print(queue)

medium

A. [30, 20, 10]

B. [10, 20, 30]

C. [10, 30, 20]

D. Error

Solution

1. Step 1: Trace each enqueue operation

   Appending 10, then 20, then 30 adds elements in order at the end.

2. Step 2: Check final queue state

   The list will be [10, 20, 30] after all appends.

3. Final Answer:

   [10, 20, 30] -> Option B

4. Quick Check:

   Appending in order = [10, 20, 30] [OK]

Hint: Appending adds elements in order at the end [OK]

Common Mistakes:

• Reversing the order of elements
• Thinking append adds at front
• Expecting an error due to empty list

4. Identify the error in the following enqueue implementation for a queue using a Python list:

def enqueue(queue, item):
    queue.insert(item)

q = [1, 2, 3]
enqueue(q, 4)
print(q)

medium

A. insert() requires two arguments: index and item

B. insert() cannot be used on lists

C. enqueue should use append() instead of insert()

D. queue is not defined

Solution

1. Step 1: Check insert() method usage

   Python list's insert() needs two arguments: position index and the item.

2. Step 2: Identify missing argument

   The code calls insert(item) with only one argument, causing an error.

3. Final Answer:

   insert() requires two arguments: index and item -> Option A

4. Quick Check:

   insert() needs index and item [OK]

Hint: insert() needs index and item arguments [OK]

Common Mistakes:

• Using insert() with one argument
• Confusing insert() with append()
• Ignoring Python method argument requirements

5. You have a queue implemented as a Python list. You want to enqueue multiple elements [7, 8, 9] at once while preserving order. Which code snippet correctly enqueues all elements in order?

hard

A. queue.extend([7, 8, 9])

B. for x in [7, 8, 9]:
queue.insert(0, x)

C. queue.append([7, 8, 9])

D. queue = [7, 8, 9] + queue

Solution

1. Step 1: Understand enqueue multiple elements

   Enqueue adds elements at the end preserving order.

2. Step 2: Analyze each option

   for x in [7, 8, 9]: queue.insert(0, x) inserts at front reversing order; queue.append([7, 8, 9]) appends list as single element; queue = [7, 8, 9] + queue prepends elements; queue.extend([7, 8, 9]) extends list adding elements in order at end.

3. Final Answer:

   queue.extend([7, 8, 9]) -> Option A

4. Quick Check:

   extend() adds multiple elements at end [OK]

Hint: Use extend() to add multiple elements at queue end [OK]

Common Mistakes:

• Using append() which nests list inside queue
• Using insert(0, x) which reverses order
• Prepending elements instead of appending