Grouping Experiments - codepath/compsci_guides GitHub Wiki

TIP102 Unit 6 Session 1 Advanced (Click for link to problem statements)

Problem Highlights

• 💡 Difficulty: Medium
• ⏰ Time to complete: 20-30 mins
• 🛠️ Topics: Linked Lists, Reorganization

1: U-nderstand

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

• Q: What does the problem ask for?
  • A: The problem asks to reorganize a linked list so that all nodes in odd positions come before nodes in even positions while maintaining their relative order.
• Q: How should the nodes be reorganized?
  • A: Nodes in odd positions should be grouped together, followed by nodes in even positions.

HAPPY CASE
Input: experiment_results1 = Node(1, Node(2, Node(3, Node(4, Node(5)))))
Output: 1 -> 3 -> 5 -> 2 -> 4
Explanation: All nodes in odd positions are grouped before nodes in even positions.

HAPPY CASE
Input: experiment_results2 = Node(2, Node(1, Node(3, Node(5, Node(6, Node(4, Node(7)))))))
Output: 2 -> 3 -> 6 -> 7 -> 1 -> 5 -> 4
Explanation: All nodes in odd positions are grouped before nodes in even positions.

EDGE CASE
Input: exp_results = Node(1)
Output: 1
Explanation: A single-node list remains unchanged.

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 involving Reorganization, we want to consider the following approaches:

• Two Pointers: Use two pointers to separate the odd and even nodes and then link them accordingly.

3: P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: We will use two pointers, one for odd positions and one for even positions. As we traverse the list, we will rearrange the links to separate the odd and even nodes. Finally, we will link the last odd node to the head of the even list.

1) If the list is empty or has only one node, return the list as is.
2) Initialize two pointers, `odd` and `even`, pointing to the first and second nodes, respectively.
3) Traverse the list:
    a) Link the current odd node to the next odd node.
    b) Move the odd pointer to the next odd node.
    c) Link the current even node to the next even node.
    d) Move the even pointer to the next even node.
4) After the loop ends, link the last odd node to the head of the even list.
5) Return the head of the reorganized list.

⚠️ Common Mistakes

• Forgetting to handle edge cases, such as a single-node list or a list with no even nodes.
• Incorrectly linking the nodes, leading to loss of nodes or incorrect order.

4: I-mplement

Implement the code to solve the algorithm.

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

def odd_even_experiments(exp_results):
    if not exp_results or not exp_results.next:
        return exp_results
    
    odd = exp_results
    even = exp_results.next
    even_head = even  # Save the head of the even list
    
    while even and even.next:
        odd.next = even.next  # Link current odd node to the next odd node
        odd = odd.next  # Move odd pointer to the next odd node
        even.next = odd.next  # Link current even node to the next even node
        even = even.next  # Move even pointer to the next even node
    
    odd.next = even_head  # Link the last odd node to the head of the even list
    
    return exp_results

5: R-eview

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

• Example: Use the provided experiment_results1 and experiment_results2 linked lists to verify that the function correctly rearranges the list.

6: E-valuate

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

Assume N represents the number of nodes in the linked list.

• Time Complexity: O(N) because each node is visited exactly once.
• Space Complexity: O(1) because the algorithm uses a constant amount of extra space for pointers.