Singly linked list structure in Data Structures Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a singly linked list?

A singly linked list is a collection of nodes where each node contains data and a reference (or link) to the next node in the sequence. It allows sequential access from the first node to the last.

beginner

What are the main parts of a node in a singly linked list?

Each node has two parts: data, which stores the value, and next, which is a reference to the next node in the list or null if it is the last node.

beginner

How do you traverse a singly linked list?

Start at the first node (called the head), then follow each node's next reference until you reach a node whose next is null, which means the end of the list.

intermediate

What is the difference between a singly linked list and a doubly linked list?

A singly linked list nodes have one link to the next node only, while doubly linked list nodes have two links: one to the next node and one to the previous node, allowing backward traversal.

intermediate

Why might you choose a singly linked list over an array?

A singly linked list allows easy insertion and deletion of nodes without shifting elements, and it can grow or shrink dynamically without needing a fixed size like arrays.

What does each node in a singly linked list contain?

A Data and references to next and previous nodes

B Data and a reference to the next node

C Only data

D Only a reference to the next node

Each node contains data and a reference (link) to the next node only.

How do you know when you have reached the end of a singly linked list?

A When the node points back to the head

B When the data is null

C When the next reference is null

D When the previous reference is null

The last node's next reference is null, indicating the end of the list.

Which operation is easier in a singly linked list compared to an array?

A Sorting the elements

B Accessing an element by index

C Random access

D Inserting a new element in the middle

Inserting in the middle is easier because you only change links without shifting elements.

What is the first node in a singly linked list called?

A Head

B Tail

C Root

D Start

The first node is called the head.

Can you traverse a singly linked list backwards easily?

A No, because nodes only link forward

B Yes, by following previous links

C Yes, by reversing the data

D No, because it has no links

Singly linked lists only have links to the next node, so backward traversal is not straightforward.

Explain the structure of a singly linked list and how nodes are connected.

Think about how each node points to the next one.

Describe the advantages of using a singly linked list compared to an array.

Consider how adding or removing elements works.

Practice

1. What does each node in a singly linked list contain?

easy

A. Only data without any pointers

B. Data and pointers to both next and previous nodes

C. Data and a pointer to the next node

D. Only a pointer to the previous node

Solution

1. Step 1: Understand node structure in singly linked list

   Each node stores some data and a pointer to the next node only, not to the previous one.

2. Step 2: Compare options with definition

   Data and a pointer to the next node matches the definition exactly. Data and pointers to both next and previous nodes describes a doubly linked list. Options C and D miss the pointer to the next node.

3. Final Answer:

   Data and a pointer to the next node -> Option C

4. Quick Check:

   Node = data + next pointer [OK]

Hint: Remember: singly linked means one pointer per node [OK]

Common Mistakes:

• Confusing singly linked with doubly linked list
• Thinking nodes have no pointers
• Assuming pointer to previous node exists

2. Which of the following is the correct way to represent the head of a singly linked list in code?

easy

A. head = None

B. head = head.next

C. head = head.prev

D. head = data

Solution

1. Step 1: Understand the head pointer initialization

   The head points to the first node or is None if the list is empty.

2. Step 2: Analyze options

   head = None correctly initializes head as None for an empty list. head = head.next tries to move head to next node, which is invalid if list is empty. head = head.prev uses prev pointer which does not exist in singly linked list. head = data assigns data to head, which is incorrect.

3. Final Answer:

   head = None -> Option A

4. Quick Check:

   Empty list head = None [OK]

Hint: Head starts as None when list is empty [OK]

Common Mistakes:

• Using prev pointer in singly linked list
• Assigning data directly to head
• Moving head before list has nodes

3. Given the following singly linked list nodes:

node1 = Node(data=10, next=node2)
node2 = Node(data=20, next=node3)
node3 = Node(data=30, next=None)
head = node1

What is the data value of head.next.next?

medium

A. 20

B. 30

C. 10

D. None

Solution

1. Step 1: Trace the linked nodes from head

   head points to node1 with data 10. head.next points to node2 with data 20. head.next.next points to node3 with data 30.

2. Step 2: Identify the data at head.next.next

   The data at node3 is 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   head.next.next.data = 30 [OK]

Hint: Follow next pointers step-by-step [OK]

Common Mistakes:

• Confusing node positions
• Assuming next of last node is not None
• Mixing data values with node references

4. Identify the error in this singly linked list insertion code snippet:

new_node = Node(data=5)
new_node.next = head
head = new_node.next

medium

A. new_node.next should be None

B. No error, code is correct

C. Data assignment to new_node is missing

D. head should be assigned to new_node, not new_node.next

Solution

1. Step 1: Understand insertion at head

   To insert new_node at the start, new_node.next should point to current head, then head should be updated to new_node.

2. Step 2: Analyze the code error

   The code sets new_node.next = head correctly, but then sets head = new_node.next, which points to old head, not new_node. This loses the new node as head.

3. Final Answer:

   head should be assigned to new_node, not new_node.next -> Option D

4. Quick Check:

   head = new_node after insertion [OK]

Hint: After linking, head must point to new_node [OK]

Common Mistakes:

• Assigning head to new_node.next instead of new_node
• Forgetting to link new_node.next to old head
• Not updating head pointer after insertion

5. You want to reverse a singly linked list with nodes:

head -> node1 -> node2 -> node3 -> None

Which sequence of pointer changes correctly reverses the list?

hard

A. Change each node's next pointer to the previous node, then update head to last node

B. Swap data values between nodes without changing pointers

C. Set head to None and create a new list with reversed data

D. Change each node's next pointer to None

Solution

1. Step 1: Understand reversal of singly linked list

   Reversing means changing each node's next pointer to point to the previous node instead of the next.

2. Step 2: Identify correct pointer update sequence

   Change each node's next pointer to the previous node, then update head to last node correctly describes reversing pointers and updating head to the last node. Swap data values between nodes without changing pointers only swaps data, not pointers. Set head to None and create a new list with reversed data discards the list unnecessarily. Change each node's next pointer to None breaks links without reversing.

3. Final Answer:

   Change each node's next pointer to the previous node, then update head to last node -> Option A

4. Quick Check:

   Reverse = reverse pointers + update head [OK]

Hint: Reverse pointers, then reset head to last node [OK]

Common Mistakes:

• Swapping data instead of pointers
• Not updating head after reversal
• Breaking links without reconnecting