DSA interview question

Add Two Numbers Represented by Linked Lists

Medium · Data structures · Topics: linkedlist, math

The problem

A calculator for arbitrarily large integers stores each number as a linked list of digits in reverse order, least significant digit first, so 2 -> 4 -> 3 is 342. Adding two such numbers is schoolbook addition, and the reverse order is exactly what makes it easy: walk both lists together from the head, add digit by digit with a carry, and build the result list as you go. The only details are lists of different lengths and a final carry that adds one extra digit.

Example: 2 -> 4 -> 3 (342) + 5 -> 6 -> 4 (465) → 7 -> 0 -> 8 (807).

Function signature

Node* addTwoNumbers(Node* l1, Node* l2);  // digits in reverse order; returns a new list

Test cases

Input `(l1, l2)`	Output	Explanation
`(2 -> 4 -> 3, 5 -> 6 -> 4)`	`7 -> 0 -> 8`	342 + 465 = 807
`(0, 0)`	`0`
`(9 -> 9 -> 9 -> 9 -> 9 -> 9 -> 9, 9 -> 9 -> 9 -> 9)`	`8 -> 9 -> 9 -> 9 -> 0 -> 0 -> 0 -> 1`	Different lengths and a final carry
`(1, 9 -> 9)`	`0 -> 0 -> 1`	1 + 99 = 100
`(5, 5)`	`0 -> 1`	The final carry creates a new digit

(Confirm: digits are stored least significant first; the inputs have no leading zeros except the number 0; the result is a new list.)

How interviewers phrase it

"Add two numbers stored in reverse order in linked lists" (LeetCode 2).
The forward-order version (LeetCode 445) needs stacks or reversal first.

Clarifying questions to ask

Are the digits stored in reverse (least significant first)?
Can the lists have different lengths?
Should I create a new list or reuse the input nodes?

Follow-ups to expect

"How do you handle different lengths?"
"When does the loop stop?"
"Why a dummy head?"
"What if the digits were most significant first?"
"Complexity?"

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