Question

Give an algorithm for finding the penultimate (second to last) node in a singly linked list where the last element is indicated by a null next link.

Short answer

Initialize two pointers and traverse the list until current.next is null; `prev` will be the penultimate node.

Step-by-step solution

- Initialize Pointers

Initialize two pointers, `prev` and `current`. Set `prev` to `null` and `current` to the head of the linked list.

- Traverse the List

Use a loop to traverse the linked list. At each iteration, move `prev` to `current` and `current` to `current.next`.

- Check for End Conditions

Continue the loop until `current.next` is `null`, indicating that `current` is the last node in the list. At this point, `prev` will be the second to last node.

- Return Result

Return the `prev` pointer, which now points to the penultimate node in the linked list.

Key concepts

singly linked list

A singly linked list is a type of data structure that consists of nodes. Each node contains two parts: a piece of data and a reference (or pointer) to the next node in the sequence. The list starts from a 'head' node and ends with a 'tail' node, which points to `null` to signify the end of the list. Since each node points only to the next one, traversal is typically straightforward but only progresses in one direction—from head to tail. This makes certain operations, like finding an element towards the end, require a full traversal of the list.

pointer manipulation

Pointer manipulation involves changing the pointers (or references) within the nodes of a data structure, like a singly linked list, to achieve specific operations. For example, in the context of finding the penultimate node, you can use two pointers: `prev` and `current`. Here's how it works:

• Initialize `prev` to `null` and `current` to the head of the list.
• During traversal, move `prev` to the `current` node and then `current` to `current.next`.
• Keep doing this until `current.next` is `null`, indicating that `current` is the last node.
• At this point, `prev` will be pointing to the penultimate node, which is the one you are looking for.

By manipulating these pointers correctly, you can efficiently find the desired node in a singly linked list.

loop traversal

Loop traversal involves using a loop structure to visit each node in a singly linked list. The key steps in this traversal method to find the penultimate node are:

• Start by initializing two pointers.
• Set up a loop that continues running as long as `current.next` is not `null`.
• In each iteration, update the pointers to move to the next pair of nodes.
• When the loop exits—when `current.next` is `null`—`prev` will be pointing to the second-to-last node.

Loop traversal is a fundamental method used to iterate over data structures. By understanding it, you can perform various operations efficiently on a singly linked list, such as finding, inserting, or deleting nodes.