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Leetcode 2 Add Two Numbers - Medium

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2. Add Two Numbers - Medium

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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

inputs:
2 -> 4 -> 3
5 -> 6 -> 4

outputs:
7 -> 0 -> 8

Solution:

Method DummyNode + LinkedList Traversing, Space O(N), Time O(N)

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
        dummy = ListNode(0)
        ptr = dummy
        addOn = 0
        while l1 or l2:
            l1_val = (l1.val if l1 else 0)
            l2_val = (l2.val if l2 else 0)
            curSum = l1_val + l2_val + addOn
            if curSum >= 10:
                curSum = curSum - 10
                addOn = 1
            else:
                addOn = 0
            ptr.next = ListNode(curSum)
            ptr = ptr.next            
            l1 = (l1.next if l1 else None)
            l2 = (l2.next if l2 else None)
        if  (addOn > 0):
            ptr.next = ListNode(addOn)
        return dummy.next

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