Is Plus One Linked List easy or hard?

Plus One Linked List is generally classified as a medium difficulty problem. The main challenge is correctly handling carry when multiple trailing digits are 9, especially without reversing the list or using extra memory.

Plus One Linked List Python/Java solution

Python and Java implementations typically follow the last non‑9 traversal technique. Track the rightmost non‑9 node during iteration, increment it, and reset subsequent digits to zero. This produces an O(n) time and O(1) space solution.

How to solve Plus One Linked List in O(n)?

Traverse the list and remember the last node whose value is not 9. After the traversal, increment that node by one and set all nodes after it to 0. If every digit was 9, prepend a dummy node with value 0 and perform the same update, producing a new leading 1.

What is the best approach for Plus One Linked List?

The best approach is the last non‑9 traversal method. Traverse the linked list while tracking the rightmost node whose value is not 9. Increment that node and set all following digits to 0. This solves the problem in O(n) time with O(1) extra space without reversing the list.

Is Plus One Linked List asked at Google/Amazon/Meta?

Plus One Linked List is a common linked list interview problem that appears in preparation sets used by companies like Google, Amazon, and Meta. It tests understanding of carry propagation, linked list traversal, and edge case handling such as consecutive 9s.

What data structure is used in Plus One Linked List?

The problem uses a singly linked list where each node stores a single digit of a number. Efficient solutions rely on pointer traversal, carry handling, and simple mathematical digit operations.

What is the time complexity of Plus One Linked List?

The optimal solution runs in O(n) time because you must traverse the linked list at least once to find the last non‑9 digit. Space complexity can be O(1) with an iterative traversal or O(n) if recursion is used due to the call stack.

Description Solution Video 6:01

369. Plus One Linked List

Medium61.1% acceptance2 companiesPremium elsewhere, free here

Given a non-negative integer represented as a linked list of digits, plus one to the integer.

The digits are stored such that the most significant digit is at the head of the list.

Example 1:

Input: head = [1,2,3]
Output: [1,2,4]

Example 2:

Input: head = [0]
Output: [1]

Constraints:

The number of nodes in the linked list is in the range [1, 100].
0 <= Node.val <= 9
The number represented by the linked list does not contain leading zeros except for the zero itself.