Skip and Remove Nodes in a Linked List

Unit 6 Session 2 (Click for link to problem statements)

Problem Highlights

• 💡 Difficulty: Medium
• ⏰ Time to complete: 30 mins
• 🛠️ Topics: Linked Lists, List Manipulation

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?

• Q: What if the total nodes are fewer than m?
  • A: The list should not change as there will be no n nodes to remove after skipping m nodes.

HAPPY CASE
Input: 1 -> 2 -> 3 -> 4 -> 5 -> 6, m = 2, n = 2
Output: 1 -> 2 -> 5 -> 6
Explanation: First 2 nodes are kept, next 2 nodes are removed, and the remaining nodes are kept.

EDGE CASE
Input: 1 -> 2 -> 3, m = 4, n = 1
Output: 1 -> 2 -> 3
Explanation: As `m` is larger than the total number of nodes, no nodes are deleted.

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.

This problem involves modifying a linked list by skipping certain nodes and removing others, using pointer manipulation without extra space.

3: P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Traverse the list, skip m nodes, then remove n nodes. Repeat the pattern until the end of the list.

1) Use a pointer to traverse the list.
2) Skip `m` nodes, ensuring the pointer does not reach the end of the list prematurely.
3) Remove `n` nodes, adjusting pointers to bypass them.
4) Repeat until the end of the list is reached.
5) Return the head of the modified list.

⚠️ Common Mistakes

• Incorrectly counting nodes during the skip and remove phases.
• Failing to handle cases where m or n exceeds the number of remaining nodes.

4: I-mplement

Implement the code to solve the algorithm.

def skip_and_remove(head, m, n):
    current = head
    while current:
        # Skip m nodes
        for i in range(1, m):
            if current is None:
                return head
            current = current.next

        # Now current is the last node before the n nodes to delete
        if current is None:
            return head

        # Start deletion of next n nodes
        temp = current
        for j in range(n):
            if temp.next is None:
                break
            temp = temp.next

        # Connect the current node to the node after the N deleted nodes
        if current and temp.next:
            current.next = temp.next
        current = temp.next

    return head

5: R-eview

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

• Test with multiple configurations of m and n to ensure nodes are correctly skipped and removed.
• Validate with lists shorter than m to see that no changes are made.

6: E-valuate

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

• Time Complexity: O(n) where n is the total number of nodes in the list. Each node is processed up to once.
• Space Complexity: O(1) because no additional space is used; modifications are done in-place.