# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    '''
    Time Complexity: O(max(n, m))
    Space Complexity: O(max(n, m))
    where, n and m are the number of nodes in l1 and l2 respectively.
    Solved in 20 mins, all by yourself! Good job!
    '''
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        dummy = ListNode()
        node = dummy
        carry = 0
        while l1 and l2:
            val = carry + l1.val + l2.val
            carry = 1 if val > 9 else 0
            val = val % 10
            node.next = ListNode(val)
            node = node.next
            l1 = l1.next
            l2 = l2.next
        
        r = l1 if l1 else l2
        
        while carry > 0 and r:
            val = r.val + carry
            carry = 1 if val > 9 else 0
            node.next = ListNode(val%10)
            node = node.next
            r = r.next
        if r:
            node.next = r            

        if carry > 0:
            node.next = ListNode(carry)            

        return dummy.next