Insert at Middle Specific Position in DSA Python - Build from Scratch

Insert at Middle Specific Position in a Linked List

📖 Scenario: Imagine you are managing a queue of people waiting for a ride. Sometimes, a new person arrives and wants to join the queue at a specific position in the middle, not just at the front or end.

🎯 Goal: You will build a simple linked list and learn how to insert a new node at a specific middle position.

📋 What You'll Learn

Create a linked list with nodes containing values 10, 20, 30, 40, 50

Create a variable called position with value 3

Write code to insert a new node with value 25 at the position in the linked list

Print the linked list after insertion in the format: 10 -> 20 -> 25 -> 30 -> 40 -> 50 -> None

💡 Why This Matters

🌍 Real World

Linked lists are used in real-world applications like music playlists, undo functionality in apps, and managing queues where insertion order matters.

💼 Career

Understanding linked list insertion is fundamental for software developers working with data structures, improving problem-solving and coding interview skills.

1

Create the initial linked list

Create a linked list with nodes containing values 10, 20, 30, 40, 50 using the Node class and link them in order.

DSA Python

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

# Create nodes with values 10, 20, 30, 40, 50 and link them
# Your code here

Hint

Make nodes one by one and connect each node's next to the next node.

2

Set the insertion position

Create a variable called position and set it to 3 to specify where the new node will be inserted.

DSA Python

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

head = Node(10)
second = Node(20)
third = Node(30)
fourth = Node(40)
fifth = Node(50)

head.next = second
second.next = third
third.next = fourth
fourth.next = fifth

# Create variable position and set it to 3
# Your code here

Hint

Just create a variable named position and assign it the value 3.

3

Insert a new node at the specified position

Create a new node with value 25 and insert it at the position in the linked list. Use variables current and count to traverse the list until the node before the insertion point.

DSA Python

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

head = Node(10)
second = Node(20)
third = Node(30)
fourth = Node(40)
fifth = Node(50)

head.next = second
second.next = third
third.next = fourth
fourth.next = fifth

position = 3

# Insert new node with value 25 at position 3
# Your code here

Hint

Traverse the list until just before the position, then link the new node in between.

4

Print the linked list after insertion

Write code to print the linked list starting from head in the format: 10 -> 20 -> 25 -> 30 -> 40 -> 50 -> None.

DSA Python

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

head = Node(10)
second = Node(20)
third = Node(30)
fourth = Node(40)
fifth = Node(50)

head.next = second
second.next = third
third.next = fourth
fourth.next = fifth

position = 3

new_node = Node(25)
current = head
count = 1
while count < position - 1:
    current = current.next
    count += 1
new_node.next = current.next
current.next = new_node

# Print the linked list
# Your code here

Hint

Use a loop to print each node's data followed by '->', and end with 'None'.

Practice

1. What does it mean to insert an element at a specific middle position in a list?

easy

A. Add the element exactly at the given index, shifting others right

B. Replace the element at the given index with the new element

C. Add the element only if the list is empty

D. Add the element at the end of the list regardless of position

Solution

1. Step 1: Understand list insertion

   Inserting at a specific position means placing the new element at that index and moving existing elements to the right.

2. Step 2: Differentiate from replacement

   Replacing overwrites the element, but insertion shifts elements to keep all.

3. Final Answer:

   Add the element exactly at the given index, shifting others right -> Option A

4. Quick Check:

   Insert means add and shift, not replace or append [OK]

Hint: Insert shifts elements right at the given index [OK]

Common Mistakes:

• Confusing insert with replace
• Thinking insert always adds at end
• Assuming insert only works on empty lists

2. Which of the following is the correct syntax to insert the value 10 at index 2 in a Python list named lst?

easy

A. lst.append(2, 10)

B. lst.insert(2, 10)

C. lst.add(2, 10)

D. lst[2] = 10

Solution

1. Step 1: Recall Python list insertion method

   Python lists use the insert(index, value) method to add an element at a specific position.

2. Step 2: Check other options

   add is not a list method, append adds only at the end, and lst[2] = 10 replaces the element at index 2.

3. Final Answer:

   lst.insert(2, 10) -> Option B

4. Quick Check:

   Use insert(index, value) to add at position [OK]

Hint: Use insert(index, value) to add at any position [OK]

Common Mistakes:

• Using append to insert in middle
• Trying to assign with lst[2] = 10 instead of insert
• Using non-existent add method

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

lst = [1, 2, 4, 5]
lst.insert(2, 3)
print(lst)

medium

A. [1, 2, 3, 4, 5]

B. [1, 2, 4, 3, 5]

C. [3, 1, 2, 4, 5]

D. [1, 2, 4, 5, 3]

Solution

1. Step 1: Understand insert at index 2

   Index 2 means the third position (0-based). Inserting 3 here shifts 4 and 5 right.

2. Step 2: Visualize the list after insertion

   List becomes [1, 2, 3, 4, 5] after insertion.

3. Final Answer:

   [1, 2, 3, 4, 5] -> Option A

4. Quick Check:

   Insert at index 2 places 3 before 4 [OK]

Hint: Index 2 means third position, insert shifts right [OK]

Common Mistakes:

• Inserting after index instead of at index
• Confusing insert with replace
• Miscounting zero-based index

4. Find the error in the following code that tries to insert 99 at index 3:

lst = [10, 20, 30]
lst.insert(4, 99)
print(lst)

medium

A. Index 4 is out of range, causes error

B. insert method requires two arguments

C. No error; 99 is added at the end

D. lst should be converted to a set before insert

Solution

1. Step 1: Check index validity for insert

   Python's insert allows index beyond list length; it adds element at the end.

2. Step 2: Confirm behavior of insert with large index

   Since index 4 > length 3, 99 is appended at the end without error.

3. Final Answer:

   No error; 99 is added at the end -> Option C

4. Quick Check:

   Insert with large index appends at end [OK]

Hint: Insert with index > length appends at end [OK]

Common Mistakes:

• Assuming index out of range causes error
• Confusing insert with assignment
• Thinking list must be converted to set

5. Given a list lst = [5, 10, 15, 20, 25], write code to insert the value 12 exactly in the middle of the list. What will be the list after insertion?

hard

A. [5, 10, 15, 12, 20, 25]

B. [12, 5, 10, 15, 20, 25]

C. [5, 12, 10, 15, 20, 25]

D. [5, 10, 12, 15, 20, 25]

Solution

1. Step 1: Calculate middle index of the list

   List length is 5, middle index is length // 2 = 2.

2. Step 2: Insert 12 at index 2

   Using lst.insert(2, 12) places 12 between 10 and 15.

3. Final Answer:

   [5, 10, 12, 15, 20, 25] -> Option D

4. Quick Check:

   Middle index = 2, insert shifts elements right [OK]

Hint: Middle index = length // 2, insert there [OK]

Common Mistakes:

• Inserting at wrong index (like 3)
• Replacing element instead of inserting
• Not calculating middle index correctly