Is Add Two Numbers II easy or hard?

Add Two Numbers II is considered a medium difficulty problem. The arithmetic logic is straightforward, but handling forward-order linked lists requires an extra idea such as stacks or reversing the lists.

Add Two Numbers II Python/Java solution

Typical Python or Java solutions use either two stacks or linked list reversal. Both implementations iterate through digits, maintain a carry value, and construct the resulting linked list node by node in O(n) time.

How to solve Add Two Numbers II in O(n)?

Traverse each list once to either reverse it or push digits into stacks. Then process digits from least significant to most significant while maintaining a carry value. Each step performs constant work, so the entire algorithm finishes in O(n) time.

What is the best approach for Add Two Numbers II?

The stack-based approach is generally considered the best interview solution. Push digits from both linked lists into stacks, pop them to simulate reverse traversal, and build the result from the front while maintaining carry. This runs in O(n) time with O(n) extra space and keeps the original lists unchanged.

Is Add Two Numbers II asked at Google/Amazon/Meta?

Add Two Numbers II and its variant "Add Two Numbers" frequently appear in interviews at companies like Amazon, Google, Meta, and Microsoft. The problem tests linked list manipulation, stack usage, and handling carry during digit addition.

What data structure is used in Add Two Numbers II?

The primary data structures are singly linked lists and stacks. Linked lists store the digits in forward order, while stacks allow reverse processing of digits so addition can start from the least significant position.

What is the time complexity of Add Two Numbers II?

Both common approaches run in O(n) time where n is the number of nodes in the longer list. Each digit is processed once during addition. The stack method uses O(n) space while the linked list reversal approach can achieve O(1) auxiliary space.

Description Solution Video 28:30

445. Add Two Numbers II

Medium63.1% acceptance9 companies

You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example 1:

Input: l1 = [7,2,4,3], l2 = [5,6,4]
Output: [7,8,0,7]

Example 2:

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [8,0,7]

Example 3:

Input: l1 = [0], l2 = [0]
Output: [0]

Constraints:

The number of nodes in each linked list is in the range [1, 100].
0 <= Node.val <= 9
It is guaranteed that the list represents a number that does not have leading zeros.

Follow up: Could you solve it without reversing the input lists?