Is Double a Number Represented as a Linked List easy or hard?

The problem is rated Medium because handling carry in a forward-order linked list requires careful pointer management. The arithmetic itself is simple, but implementing correct carry propagation without reversing the list can be tricky.

Double a Number Represented as a Linked List Python/Java solution

Most solutions follow the same logic across languages. Traverse the linked list, double each digit, and manage carry propagation. Python, Java, C++, and JavaScript implementations typically use either a stack-based simulation or an in-place two-pointer technique with O(n) time complexity.

How to solve Double a Number Represented as a Linked List in O(n)?

Traverse the linked list while doubling each digit and tracking carry. Use a dummy head and remember the last node whose value is not 9. When a carry occurs, increment that node and reset intermediate digits to zero. This allows correct carry propagation in a single pass with O(n) time and O(1) space.

What is the best approach for Double a Number Represented as a Linked List?

The two-pointer carry propagation technique is generally considered the best approach. It scans the linked list once, tracks the last non‑9 digit, and handles carry without reversing the list or using extra memory. This achieves O(n) time and O(1) space complexity, which is optimal for this problem.

Is Double a Number Represented as a Linked List asked at Google/Amazon/Meta?

Linked list arithmetic problems similar to this appear in interviews at large tech companies including Amazon, Google, and Meta. Variants such as adding numbers represented by linked lists or handling digit carry are common interview patterns.

What data structure is used in Double a Number Represented as a Linked List?

The primary data structure is a singly linked list where each node stores one digit of the number. Some implementations also use a stack to process digits from right to left, while the optimal approach uses pointer tracking within the linked list.

What is the time complexity of Double a Number Represented as a Linked List?

The optimal solution runs in O(n) time where n is the number of nodes in the linked list. Each node is processed at most once during traversal. Stack-based implementations also run in O(n) time but require additional O(n) space.

Description Hints 3 Solution Video 43:12

2816. Double a Number Represented as a Linked List

Medium61.4% acceptance3 companies

You are given the head of a non-empty linked list representing a non-negative integer without leading zeroes.

Return the head of the linked list after doubling it.

Example 1:

Input: head = [1,8,9]
Output: [3,7,8]
Explanation: The figure above corresponds to the given linked list which represents the number 189. Hence, the returned linked list represents the number 189 * 2 = 378.

Example 2:

Input: head = [9,9,9]
Output: [1,9,9,8]
Explanation: The figure above corresponds to the given linked list which represents the number 999. Hence, the returned linked list reprersents the number 999 * 2 = 1998.

Constraints:

The number of nodes in the list is in the range [1, 10⁴]
0 <= Node.val <= 9
The input is generated such that the list represents a number that does not have leading zeros, except the number 0 itself.

Asked at 3 companies

Amazon Microsoft

Google

Topics

Hints open one at a time

Hint 1

Traverse the linked list from the least significant digit to the most significant digit and multiply each node's value by 2

Hint 2

Handle any carry-over digits that may arise during the doubling process.

Hint 3

If there is a carry-over digit on the most significant digit, create a new node with that value and point it to the start of the given linked list and return it.

43:12

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