Add Two Numbers

Step0 / 18

PhaseInitialize L1

L1 Length3

L2 Length3

Code

1def addTwoNumbers(l1, l2):
2    dummy = ListNode(0)
3    current = dummy
4    carry = 0
5 
6    while l1 is not None or l2 is not None:
7        val1 = l1.val if l1 is not None else 0
8        val2 = l2.val if l2 is not None else 0
9 
10        total = val1 + val2 + carry
11        carry = total // 10
12        digit = total % 10
13 
14        current.next = ListNode(digit)
15        current = current.next
16 
17        if l1 is not None:
18            l1 = l1.next
19        if l2 is not None:
20            l2 = l2.next
21 
22    if carry > 0:
23        current.next = ListNode(carry)
24 
25    return dummy.next

Create dummy head

Linked Lists

INIT L1

Step

0 / 18

Phase

Initialize L1

Carry

List 1

List 2

Result

Progress

Processing

Result So Far

[Empty]

Pointer State

Pointers

current

null

carry

Key Insight

Carry propagates to the next digit. If carry remains after processing all nodes, add it as a final node.

Explanation

Init

Initialize Pointers

Set up pointers for both input lists and the result list.

💡 Tip

We'll traverse both lists simultaneously.