445.md

445 Add Two Numbers II

Description

link

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Solution

• See Code

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Code

O(n)

"""
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
"""

class Solution:
    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
        s1 = 0
        s2 = 0

        while l1:s1 *= 10; s1 += l1.val; l1 = l1.next
        while l2: s2 *= 10; s2 += l2.val; l2 = l2.next

		# take the sum and reconstruct the number from tail to head, because it's easier
		# to isolate and chop off the little digits with modulus and division.
        
        s3 = s1 + s2
        tail = None
        head = None
        
        while s3 > 0: head = ListNode(s3 % 10); head.next = tail; tail = head; s3 //= 10
            
        return head if head else ListNode(0)