DSA TrainerAdd Two Numbers
Two non-empty linked lists represent two non-negative integers with digits stored in reverse order. Add the two numbers and return the sum as a linked list (also in reverse order).
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Problem
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.
Input
Two linked lists `l1` and `l2`, each representing a non-negative integer with digits in reverse order (ones place at the head).
Output
A linked list representing the sum, with digits in reverse order.
Examples
Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
342 + 465 = 807. Stored as 7→0→8.
Input: l1 = [0], l2 = [0]
Output: [0]
Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]
The brute-force approach
Convert both lists to integers, add them, convert back to a linked list in reverse order.
num1 = int from l1; num2 = int from l2
total = num1 + num2
build linked list from digits of total in reverseO(max(m,n)) but converts to/from integers. The direct digit-by-digit approach avoids the conversion and handles arbitrarily large numbers.
Spotting the pattern
This is a Linked List problem. The key question to ask yourself:
When one list runs out before the other, what values do you use for the exhausted list's current digit in subsequent steps?
Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.
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