[LeetCode] Challenge log 2

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2. Add Two Numbers
  1. Use recursion to process node by node.
  2. When there is carry, create a new node with val = 1 to add on.

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 contain a single digit. Add the two numbers and return it as a linked list.

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

Example

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Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.
Soulution:
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# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if l1 is None: return l2
        if l2 is None: return l1
        
        val = l1.val + l2.val 
        if val >= 10: 
            val -= 10
            l1.val = val
            l1.next = self.addTwoNumbers(self.addTwoNumbers(l1.next, ListNode(1)), l2.next)
        else:
            l1.val = val
            l1.next = self.addTwoNumbers(l1.next, l2.next)
        
        return l1
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