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77 lines
2.1 KiB
Plaintext
77 lines
2.1 KiB
Plaintext
---
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title: '2. Add Two Numbers'
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description: You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list
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icon: line-dot-left-horizontal
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sidebar:
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label: 'Add Two Numbers'
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badge: 'Medium'
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---
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<Badge variant="accent">Linked List</Badge>
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### Example 1:
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- Input: `l1 = [2,4,3], l2 = [5,6,4]`
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- Output: `[7,0,8]`
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- Explanation: `342 + 465 = 807`.
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### Example 2:
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- Input: `l1 = [0], l2 = [0]`
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- Output: `[0]`
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### Example 3:
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- Input: `l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]`
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- Output: `[8,9,9,9,0,0,0,1]`
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### Constraints:
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- The number of nodes in each linked list is in the range [1, 100].
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- `0 <= Node.val <= 9`
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- It is guaranteed that the list represents a number that does not have leading zeros.
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## Approach
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```mermaid
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flowchart TD
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S(["addTwoNumbers(l1, l2)"]) --> I["dummy = ListNode(), carry = 0, curr = dummy — dummy.next will be the head"]
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I --> W{"l1 or l2 or carry?"}
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W -- no --> E(["return dummy.next"])
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W -- yes --> A["s = (l1.val if l1 else 0) + (l2.val if l2 else 0) + carry — a missing digit counts as 0"]
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A --> D["carry, val = divmod(s, 10)"]
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D --> N["curr.next = ListNode(val), curr = curr.next"]
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N --> V["l1 = l1.next if l1 else None, l2 = l2.next if l2 else None"]
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V --> W
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```
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## Solution
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```py
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# Definition for singly-linked list.
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# class ListNode:
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# def __init__(self, val=0, next=None):
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# self.val = val
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# self.next = next
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class Solution:
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def addTwoNumbers(
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self, l1: Optional[ListNode], l2: Optional[ListNode]
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) -> Optional[ListNode]:
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dummy = ListNode()
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carry = 0
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curr = dummy
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while l1 or l2 or carry:
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s = (l1.val if l1 else 0) + (l2.val if l2 else 0) + carry
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carry, val = divmod(s, 10)
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curr.next = ListNode(val)
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curr = curr.next
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l1 = l1.next if l1 else None
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l2 = l2.next if l2 else None
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return dummy.next
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```
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## Explanation
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<YouTube id="wgFPrzTjm7s" title="Add Two Numbers - Leetcode 2 - Python" />
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