Create Double Links

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Unit 5 Session 2 (Click for link to problem statements)

TIP102 Unit 5 Session 2 Standard (Click for link to problem statements)

Problem Highlights

• 💡 Difficulty: Easy
• ⏰ Time to complete: 5-10 mins
• 🛠️ Topics: Doubly Linked Lists, Node Class Modification

1: U-nderstand

Understand what the interviewer is asking for by using test cases and questions about the problem.

• Established a set (2-3) of test cases to verify their own solution later.
• Established a set (1-2) of edge cases to verify their solution handles complexities.
• Have fully understood the problem and have no clarifying questions.
• Have you verified any Time/Space Constraints for this problem?

• What happens if there is only one node?
  • The prev and next attributes of the single node should both be None.

HAPPY CASE
Input: head = Node("Isabelle"), tail = Node("K.K. Slider")
Output: head.next = tail, tail.prev = head
Explanation: The `next` pointer of the head points to the tail, and the `prev` pointer of the tail points to the head.

EDGE CASE
Input: head = Node("Isabelle")
Output: head.next = None, head.prev = None
Explanation: If there is only one node, both `next` and `prev` pointers should be `None`.

2: M-atch

Match what this problem looks like to known categories of problems, e.g. Linked List or Dynamic Programming, and strategies or patterns in those categories.

For Linked List problems, we want to consider the following approaches:

• Modification of node structure to support doubly linked lists
• Handling pointers for next and prev attributes

3: P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Modify the Node class to include a prev attribute and link the nodes to form a doubly linked list.

1) Update the `Node` class to include a `prev` parameter in the constructor.
2) Link the `next` pointer of the head node to the tail node.
3) Link the `prev` pointer of the tail node to the head node.

⚠️ Common Mistakes

• Forgetting to initialize the prev attribute, leading to errors when trying to set the prev pointer.

4: I-mplement

Implement the code to solve the algorithm.

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

# Create the nodes
head = Node("Isabelle")
tail = Node("K.K. Slider")

# Link the nodes to form a doubly linked list
head.next = tail
tail.prev = head

5: R-eview

Review the code by running specific example(s) and recording values (watchlist) of your code's variables along the way.

• Check the values of next and prev pointers for both head and tail nodes to ensure the doubly linked list is correctly formed.

Example:

print(head.value, "<->", head.next.value)
print(tail.prev.value, "<->", tail.value)


Expected Output:
Isabelle <-> K.K. Slider
Isabelle <-> K.K. Slider

6: E-valuate

Evaluate the performance of your algorithm and state any strong/weak or future potential work.

• Time Complexity: O(1) because we are only setting pointers.
• Space Complexity: O(1) because we are only storing references to nodes.