Insert Node as Second Element - codepath/compsci_guides GitHub Wiki

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

U-nderstand

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

• What should occur if there is only one node in the linked list when the function is called?
  • The new node should become the second node in the list, and the existing node (head) should link to it directly.

P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Add a new node immediately after the head of the linked list, updating the links to incorporate the new node as the second node in the list.

1) Create a new node with the specified value `val`.
2) Link the new node to the current second node in the list (`head.next`).
3) Set `head.next` to point to the new node, inserting it as the second node.
4) Return the original head of the list, which remains the first node.

⚠️ Common Mistakes

• Incorrectly linking the new node which could either bypass the original second node or disrupt the list.
• Returning the new node instead of the original head, which would misrepresent the start of the list.

I-mplement

def add_second(head, val):
    # Create the new node that needs to be added
    new_node = Node(val)
    
    # Insert the new node as the second node
    new_node.next = head.next
    head.next = new_node
    
    # Return the unchanged head of the list
    return head