LeetCode: 2. Add Two Numbers

2026-02-27

Let’s solve this problem in two ways: a while loop and recursion.

LeetCode: 2. Add Two Numbers

LeetCode: 2. Add Two Numbers

Summary

Add two numbers represented by linked lists and build a new linked list for the result.

Approach

Pick nodes from the heads of both linked lists. Calculate the current digit and the carry for the next digit. Continue until both nodes are null.

Function

Initialization:

  • carry = 0
  • l1, l2 (current nodes)
  • new_num = ListNode(0)
  • tail = new_num

Main loop:

Use a while loop while either l1 or l2 is not null.

  • Pick the nodes from l1 and l2.
  • Get values from the nodes. If a node is null, use zero.
  • Add l1.val + l2.val + carry = total.
  • Calculate digit = total % 10 and carry = total // 10.
  • Append the new node to new_num.
  • Move to the next nodes (l1 = l1.next, l2 = l2.next) when available.

Complexity

Time Complexity: O(max(L1, L2)), where L1 is the length of l1 and L2 is the length of l2. Space Complexity: O(1) auxiliary space.

Approach2

Recursive approach.

Function

Skipped, because it is almost the same.

Complexity

Time & Space Complexity: O(max(L1, L2))

The space complexity is worse than Approach 1.