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:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

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

class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        p = l1
        q = l2
        carry = 0
        head = ListNode(0)
        current = head
        while p is not None or q is not None:
            x = p.val if p != None else 0
            y = q.val if q != None else 0
            
            if p != None: 
                p = p.next
            if q != None: 
                q = q.next
            
            sum = x+y+carry
            carry = sum/10
            
            current.val = sum%10
            if p is not None and q is not None:
                current.next = ListNode(0)
            else: 
                current.next = None
            current = current.next
        if carry == 1:
            current.next = ListNode(1)
            
        return head