2. Add Two Numbers

Approach 1: Iterate through both of the lists simultaneously

# 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 not l1 or not l2: return l1 or l2
        head = ListNode('a')
        a = head
        carry = 0
        while l1 and l2:
            a.next = ListNode((l1.val + l2.val + carry)%10)
            carry = (l1.val + l2.val + carry)//10
            a = a.next
            l1 = l1.next
            l2 = l2.next
        while l1:
            a.next = ListNode((l1.val + carry)%10)
            carry = (l1.val + carry)//10
            a = a.next
            l1 = l1.next
        while l2:
            a.next = ListNode((l2.val + carry)%10)
            carry = (l2.val + carry)//10
            a = a.next
            l2 = l2.next
        if carry != 0:
            a.next = ListNode(carry)
        return head.next

In this we wil iterate through each of these lists simultaneously and

Time Complexity: O(N), where N = max(len(l1), len(l2))

Space Complexity: O(N), where N = max(len(l1), len(l2))