Node Structure and Pointer Design in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a node in data structures?

A node is a basic unit that contains data and references (pointers) to other nodes. It helps build structures like linked lists and trees.

beginner

What is a pointer in the context of nodes?

A pointer is a reference inside a node that points to another node's location, allowing connection between nodes.

intermediate

How does a singly linked list node differ from a doubly linked list node?

A singly linked list node has one pointer to the next node, while a doubly linked list node has two pointers: one to the next node and one to the previous node.

beginner

Why do we use pointers in node structures?

Pointers allow nodes to connect dynamically, enabling flexible data structures that can grow or shrink without fixed size.

beginner

What happens if a node's pointer is set to null (or None in Python)?

It means the node does not point to any other node, often marking the end of a structure like a linked list.

What does a node typically contain?

A Only data

B Data and pointers

C Only pointers

D Functions

A node contains data and pointers to connect with other nodes.

In a singly linked list, how many pointers does each node have?

A Three

B Two

C None

D One

Each node in a singly linked list has one pointer to the next node.

What does a pointer set to None indicate in Python node structures?

A End of the list or no connection

B Middle of the list

C Error in the node

D Start of the list

A pointer set to None means no next node, marking the end or no connection.

Why are pointers important in linked data structures?

A They store data

B They increase memory size

C They connect nodes dynamically

D They sort the data

Pointers connect nodes dynamically, allowing flexible data structures.

Which node type has pointers to both next and previous nodes?

A Doubly linked list node

B Binary tree node

C Singly linked list node

D Stack node

Doubly linked list nodes have pointers to both next and previous nodes.

Explain the structure of a node and the role of pointers in linked data structures.

Think about how nodes connect to form lists or trees.

Describe the difference between singly and doubly linked list nodes in terms of pointers.

Focus on how many pointers each node has and what they point to.

Practice

1. What does the next pointer in a linked list node represent?

easy

A. It points to the previous node in the list

B. It stores the data value of the node

C. It points to the next node in the list

D. It stores the size of the list

Solution

1. Step 1: Understand the role of next pointer

   The next pointer in a linked list node is used to connect the current node to the following node in the sequence.

2. Step 2: Differentiate from other node parts

   The data part stores the value, and next specifically points to the next node, not previous or size.

3. Final Answer:

   It points to the next node in the list -> Option C

4. Quick Check:

   next pointer = next node address [OK]

Hint: Remember: next always leads forward in the list [OK]

Common Mistakes:

• Confusing next with data storage
• Thinking next points backward
• Assuming next stores list size

2. Which of the following is the correct way to define a simple node class in Python with a data attribute and a next pointer?

easy

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

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

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

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

Solution

1. Step 1: Check correct attribute assignment

   class Node: def __init__(self, data): self.data = data self.next = None correctly assigns self.data = data and initializes self.next = None to indicate no next node yet.

2. Step 2: Identify errors in other options

   class Node: def __init__(self, data): data = self.data next = None assigns backwards, class Node: def __init__(self): self.data = None self.next = None misses data parameter, class Node: def __init__(self, data, next): self.data = data next = None does not assign self.next properly.

3. Final Answer:

   class Node: def __init__(self, data): self.data = data self.next = None -> Option A

4. Quick Check:

   Correct attribute setup = class Node: def __init__(self, data): self.data = data self.next = None [OK]

Hint: Use self.attribute = value inside __init__ [OK]

Common Mistakes:

• Not using self to assign attributes
• Forgetting to initialize next to None
• Swapping assignment order

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

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

head = Node(10)
second = Node(20)
head.next = second
print(head.next.data)

medium

A. 20

B. Error

C. None

D. 10

Solution

1. Step 1: Understand node creation and linking

   Two nodes are created: head with data 10, second with data 20. Then, head.next points to second.

2. Step 2: Trace the print statement

   head.next.data accesses the data of the node after head, which is second, so it prints 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   head.next.data = 20 [OK]

Hint: Follow the next pointer to get next node's data [OK]

Common Mistakes:

• Printing head.data instead of head.next.data
• Confusing next pointer with data
• Expecting None because of uninitialized pointer

4. Identify the error in the following code snippet for a singly linked list node:

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

node1 = Node(5)
print(node1.next)

medium

A. The class name should be lowercase

B. The data attribute is missing

C. The print statement syntax is wrong

D. The next pointer is not assigned to self

Solution

1. Step 1: Check attribute assignment in __init__

   The code assigns next = None without self., so next is a local variable, not an attribute.

2. Step 2: Understand consequence on print

   Since node1.next was never set, it raises an AttributeError or prints None if accessed differently.

3. Final Answer:

   The next pointer is not assigned to self -> Option D

4. Quick Check:

   Missing self. causes attribute error [OK]

Hint: Always use self. to create object attributes [OK]

Common Mistakes:

• Forgetting self. before next
• Assuming local variable becomes attribute
• Ignoring error from missing attribute

5. You want to create a linked list with three nodes containing values 1, 2, and 3 respectively. Which code snippet correctly links the nodes?

hard

A. n1 = Node(1)
n2 = Node(2)
n3 = Node(3)
n1.next = n3
n3.next = n2
n2.next = None

B. n1 = Node(1)
n2 = Node(2)
n3 = Node(3)
n1.next = n2
n2.next = n3
n3.next = None

C. n1 = Node(1)
n2 = Node(2)
n3 = Node(3)
n1.next = None
n2.next = n1
n3.next = n2

D. n1 = Node(1)
n2 = Node(2)
n3 = Node(3)
n3.next = n2
n2.next = n1
n1.next = None

Solution

1. Step 1: Understand correct node linking order

   To create a linked list 1 -> 2 -> 3, n1.next should point to n2, and n2.next to n3. The last node n3.next should be None.

2. Step 2: Check each option's linking

   n1 = Node(1) n2 = Node(2) n3 = Node(3) n1.next = n2 n2.next = n3 n3.next = None correctly links nodes in order. The other options link nodes in wrong order or reverse.

3. Final Answer:

   n1.next = n2; n2.next = n3; n3.next = None -> Option B

4. Quick Check:

   Correct forward linking = n1 = Node(1) n2 = Node(2) n3 = Node(3) n1.next = n2 n2.next = n3 n3.next = None [OK]

Hint: Link nodes in order, last node points to None [OK]

Common Mistakes:

• Linking nodes in reverse order
• Not setting last node's next to None
• Mixing node references