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mediumDSA

How do you add two numbers stored as linked lists?

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01

Understand the problem

Explain digit-by-digit addition with carry.

linked-listmath
02

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03

Study the solution

Walk both lists together, summing corresponding digits plus a carry, creating a new node for sum % 10 and carrying sum / 10. Continue until both lists and the carry are exhausted. A dummy head simplifies building the result. O(max(m,n)).

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04

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Digit-by-digit with carry
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class ListNode:
    def __init__(self, val, nxt=None):
        self.val, self.next = val, nxt

def add_two_numbers(l1, l2):
    dummy = tail = ListNode(0)
    carry = 0
    while l1 or l2 or carry:
        total = carry
        if l1: total += l1.val; l1 = l1.next
        if l2: total += l2.val; l2 = l2.next
        carry, digit = divmod(total, 10)
        tail.next = ListNode(digit)
        tail = tail.next
    return dummy.next


# --- demo ---  342 + 465 = 807 (digits stored reversed)
def build(vals):
    head = None
    for v in reversed(vals): head = ListNode(v, head)
    return head
def to_list(head):
    out = []
    while head: out.append(head.val); head = head.next
    return out

print(to_list(add_two_numbers(build([2, 4, 3]), build([5, 6, 4]))))   # [7, 0, 8]
05

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