Is Add Two Numbers easy or hard?

Add Two Numbers is generally classified as a Medium difficulty problem. The main challenge is correctly handling different list lengths and carry propagation while constructing the result linked list.

Add Two Numbers Python/Java solution

In Python or Java, the solution typically uses a loop that iterates through both linked lists while tracking a carry variable. A dummy head node is used to simplify result construction, and new nodes are appended as each digit of the sum is calculated.

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

Traverse both linked lists simultaneously and maintain a carry variable. At each step compute sum = digit1 + digit2 + carry, store sum % 10 as the new node, and update carry = sum // 10. Continue until both lists are exhausted and no carry remains, giving an O(n) solution where n is the longer list length.

What is the best approach for Add Two Numbers?

The standard solution uses iterative traversal of both linked lists while maintaining a carry value. Each step adds corresponding digits and stores the result digit in a new node. This approach runs in O(n) time and O(1) extra space, making it the most efficient and the most commonly expected answer in coding interviews.

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

Add Two Numbers is a classic linked list problem frequently reported in interviews at companies like Amazon, Google, Meta, and Microsoft. It tests pointer manipulation, carry handling, and understanding of linked list traversal.

What data structure is used in Add Two Numbers?

The core data structure is a singly linked list where each node stores a single digit. The algorithm traverses the lists node by node while constructing a new result linked list. Basic arithmetic operations are used to manage the carry between digits.

What is the time complexity of Add Two Numbers?

The time complexity is O(max(m,n)), where m and n are the lengths of the two linked lists. Each node from both lists is visited once to compute the digit sum and carry. Space complexity is O(1) for the iterative approach, excluding the output list.

Description Solution Video 9:33

2. Add Two Numbers

Medium49.3% acceptance37 companies

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, 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 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

Example 2:

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

Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]

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.