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LeetCode solutions

2871. Double a Number Represented as a Linked List

My accepted Python solution to LeetCode problem 2871, Double a Number Represented as a Linked List, running in 35ms.

  • Difficulty: Medium
  • Python
  • Runtime 35ms
  • Memory 19.4MB

Read the problem on LeetCode View on GitHub

The problem statement is LeetCode’s and stays on their site. What follows is my accepted solution.

Python

Accepted on LeetCode — runtime 35ms, memory 19.4MB, accepted 2025-12-30.

python
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def doubleIt(self, head: Optional[ListNode]) -> Optional[ListNode]:
        # Reverse the linked list to process from least significant digit
        def reverse(node):
            prev = None
            while node:
                next_node = node.next
                node.next = prev
                prev = node
                node = next_node
            return prev
        
        # Reverse the list
        head = reverse(head)
        
        # Double each digit and handle carry
        carry = 0
        curr = head
        prev = None
        
        while curr:
            total = curr.val * 2 + carry
            curr.val = total % 10
            carry = total // 10
            prev = curr
            curr = curr.next
        
        # If there's still a carry, add new node
        if carry:
            prev.next = ListNode(carry)
        
        # Reverse back
        return reverse(head)

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