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Add Two Numbers – Solution & Complexity

Interview solutions
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Add Two Numbers

Solution Walkthrough

1. Recognize the Pattern

The key pattern is: Digit-by-digit simulation of grade-school addition with carry propagation. Identify the state and invariant before coding.

2. Build the Algorithm

Advance one state transition at a time. Mark or update state before exploring dependent work.

3. Check Edge Cases

Test empty or minimal input, skewed shapes, duplicates where allowed, and impossible outcomes.

4. Solution and Complexity

Time: O(max(m, n)). Space: O(max(m, n)) for the output list.

All 5 languages below run and submit against the remote judge for this problem — Java/Go/Rust use the same real ListNode structural-type support the judge added for the rest of the linked-list/tree track.

def add_two_numbers(l1: ListNode, l2: ListNode) -> ListNode:
    dummy = ListNode(0)
    current = dummy
    carry = 0

    while l1 is not None or l2 is not None or carry:
        v1 = l1.val if l1 is not None else 0
        v2 = l2.val if l2 is not None else 0
        total = v1 + v2 + carry
        carry = total // 10
        current.next = ListNode(total % 10)
        current = current.next
        l1 = l1.next if l1 is not None else None
        l2 = l2.next if l2 is not None else None

    return dummy.next

FAQ

See also

Practice Add Two Numbers online (run code in the browser)Climbing Stairs solution (easy)Count Primes solution (medium)Excel Sheet Column Number solution (easy)Factorial solution (easy)All coding interview problems
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