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

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a type of linked list where each node contains three parts: data, a pointer to the next node, and a pointer to the previous node. This allows traversal in both directions.

beginner

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

A singly linked list has nodes with only one pointer to the next node, allowing traversal in one direction. A doubly linked list has two pointers per node, enabling traversal forwards and backwards.

intermediate

What are the advantages of using a doubly linked list?

Advantages include easy backward traversal, efficient deletion of a node when given a pointer to it, and easier insertion before a given node compared to singly linked lists.

intermediate

What is a common disadvantage of doubly linked lists compared to singly linked lists?

They use more memory because each node stores an extra pointer to the previous node, and operations can be slightly slower due to managing two pointers.

intermediate

Describe the steps to insert a new node after a given node in a doubly linked list.

1. Create the new node with data.
2. Set the new node's next pointer to the given node's next.
3. Set the new node's previous pointer to the given node.
4. Update the given node's next node's previous pointer to the new node (if it exists).
5. Update the given node's next pointer to the new node.

What pointers does each node in a doubly linked list contain?

A Pointers to the next and previous nodes

B Pointer to the next node only

C Pointer to the previous node only

D No pointers, only data

Each node in a doubly linked list has two pointers: one to the next node and one to the previous node.

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

A Accessing the middle node directly

B Finding the head node

C Backward traversal

D Sorting the list

Backward traversal is easier because nodes have pointers to their previous nodes.

What is a disadvantage of doubly linked lists?

A They use more memory per node

B They cannot be traversed backwards

C They do not allow insertion

D They have no pointers

Doubly linked lists use more memory because each node stores two pointers instead of one.

If you have a pointer to a node in a doubly linked list, how can you delete it efficiently?

A By copying data from the next node

B You must start from the head to delete it

C You cannot delete nodes in doubly linked lists

D By updating the previous and next nodes' pointers to skip it

You can delete a node by adjusting the previous node's next pointer and the next node's previous pointer to bypass the node.

Which of these is NOT a part of a doubly linked list node?

A Data

B Pointer to head node

C Pointer to previous node

D Pointer to next node

Nodes do not store a pointer to the head node; they only store data and pointers to adjacent nodes.

Explain what a doubly linked list is and how it works.

Think about how nodes connect forwards and backwards.

Describe the advantages and disadvantages of using a doubly linked list.

Consider what extra pointer means for memory and functionality.

Practice

1. What is a key feature of a doubly linked list compared to a singly linked list?

easy

A. It stores data in a fixed-size array

B. Nodes only link to the next node

C. Each node has links to both the previous and next nodes

D. It uses a stack structure internally

Solution

1. Step 1: Understand node links in doubly linked lists

   A doubly linked list node contains two links: one to the previous node and one to the next node.

2. Step 2: Compare with singly linked list

   A singly linked list node only has a link to the next node, not the previous one.

3. Final Answer:

   Each node has links to both the previous and next nodes -> Option C

4. Quick Check:

   Doubly linked list = two links per node [OK]

Hint: Remember: doubly means two links per node [OK]

Common Mistakes:

• Thinking nodes only link forward
• Confusing with arrays or stacks
• Assuming fixed size storage

2. Which of the following is the correct way to update links when inserting a new node newNode after a node current in a doubly linked list?

easy

A. newNode.next = current.next; newNode.prev = current; current.next.prev = newNode; current.next = newNode;

B. newNode.prev = current.next; newNode.next = current; current.prev = newNode; current.next = newNode;

C. current.next = newNode; newNode.next = current.prev; current.prev = newNode;

D. newNode.next = current.prev; newNode.prev = current.next; current.next = newNode;

Solution

1. Step 1: Set newNode's next and prev pointers

   newNode.next should point to current.next, and newNode.prev should point to current.

2. Step 2: Update surrounding nodes to link to newNode

   current.next.prev must point back to newNode, and current.next must point to newNode.

3. Final Answer:

   newNode.next = current.next; newNode.prev = current; current.next.prev = newNode; current.next = newNode; -> Option A

4. Quick Check:

   Update newNode links, then fix neighbors [OK]

Hint: Update new node links before neighbors [OK]

Common Mistakes:

• Mixing prev and next pointers
• Not updating current.next.prev
• Assigning pointers in wrong order

3. Consider the following doubly linked list nodes and code snippet:

Node1: data=10, prev=None, next=Node2
Node2: data=20, prev=Node1, next=Node3
Node3: data=30, prev=Node2, next=None

current = Node2
print(current.prev.data, current.data, current.next.data)

What will be the output?

medium

A. None 20 30

B. 30 20 10

C. 20 20 20

D. 10 20 30

Solution

1. Step 1: Identify current node and its neighbors

   current is Node2 with data 20; prev is Node1 with data 10; next is Node3 with data 30.

2. Step 2: Print data values of prev, current, and next nodes

   Output will be 10 (prev), 20 (current), 30 (next).

3. Final Answer:

   10 20 30 -> Option D

4. Quick Check:

   Prev, current, next data = 10, 20, 30 [OK]

Hint: Prev and next data surround current data [OK]

Common Mistakes:

• Confusing prev and next nodes
• Assuming None has data
• Printing node references instead of data

4. The following code attempts to delete a node current from a doubly linked list. What is the error?

current.prev.next = current.next
current.next.prev = current.prev
current = None

medium

A. It should only update current.prev.next, not current.next.prev

B. It does not handle the case when current is the head node

C. It should update current.next.next instead of current.prev.next

D. It incorrectly sets current to None instead of deleting

Solution

1. Step 1: Analyze pointer updates for deletion

   The code updates the previous node's next pointer and the next node's prev pointer to bypass current.

2. Step 2: Check edge cases for head or tail nodes

   If current is the head (no prev), current.prev is None, so current.prev.next causes an error.

3. Final Answer:

   It does not handle the case when current is the head node -> Option B

4. Quick Check:

   Deleting head needs special handling [OK]

Hint: Check for None before updating neighbors [OK]

Common Mistakes:

• Ignoring head or tail node cases
• Assuming current is always in middle
• Confusing which pointers to update

5. You want to reverse a doubly linked list by swapping the next and prev pointers of each node. Which of the following steps is correct to achieve this?

hard

A. For each node, swap its next and prev pointers, then move to the new prev node

B. For each node, swap its next and prev pointers, then move to the new next node

C. For each node, only swap next pointers and ignore prev pointers

D. For each node, set next to None and prev to head

Solution

1. Step 1: Swap next and prev pointers for each node

   Swapping next and prev reverses the direction of links.

2. Step 2: Move to the new prev node after swapping

   After swapping, the original next is now prev, so move to node.prev to continue.

3. Final Answer:

   For each node, swap its next and prev pointers, then move to the new prev node -> Option A

4. Quick Check:

   Swap links, then follow new prev to reverse [OK]

Hint: After swapping, move to node.prev to continue [OK]

Common Mistakes:

• Moving to node.next after swapping
• Swapping only one pointer
• Setting pointers to None incorrectly