Singly linked list structure in Data Structures Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a node in a singly linked list with a value and a pointer to the next node.

Data Structures Theory

class Node:
    def __init__(self, value):
        self.value = value
        self.[1] = None

Drag options to blanks, or click blank then click option'

Atail

Bprev

Chead

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' for the pointer.

Using 'head' or 'tail' inside the node class.

In a singly linked list, each node has a pointer called next that points to the next node in the list.

2fill in blank

medium

Complete the code to create a new node and link it as the next node of the current node.

Data Structures Theory

current_node.next = [1](10)

Drag options to blanks, or click blank then click option'

ANode

BList

CLinkedList

DElement

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong class name to create a node.

Assigning a value directly instead of a node object.

To link a new node, you create an instance of the Node class and assign it to the next pointer.

3fill in blank

hard

Fix the error in the code to correctly traverse a singly linked list until the end.

Data Structures Theory

while current_node.[1] is not None:
    current_node = current_node.next

Drag options to blanks, or click blank then click option'

Aprev

Btail

Cnext

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' in the loop condition.

Checking 'head' or 'tail' which are not node pointers.

To move through a singly linked list, you follow the next pointers until you reach a node where next is None.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps each node's value to the value of its next node, or None if there is no next node.

Data Structures Theory

value_map = {node.value: node.[1].[2] if node.[1] else None for node in nodes}

Drag options to blanks, or click blank then click option'

Anext

Bvalue

Cprev

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' for the pointer.

Accessing 'head' or 'prev' which do not exist in nodes.

Each node's next pointer leads to the next node, and we access its value. If there is no next node, we use None.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps each node's value to the value of the node two steps ahead, or None if not available.

Data Structures Theory

two_ahead_map = {node.[1]: node.[2].[3].value if node.[2] and node.[2].[3] else None for node in nodes}

Drag options to blanks, or click blank then click option'

Avalue

Bnext

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up 'value' and 'next' in the blanks.

Using 'prev' which does not exist in singly linked lists.

The first blank accesses the current node's value as the key. The second blank is the next pointer to the next node. The third blank is the next pointer from that next node to the node two steps ahead. We then get the value of that node.

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