Add Two Numbers II — Python

# Time:  O(m + n)
# Space: O(m + n)

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
        """
        stk1, stk2 = [], []
        while l1:
            stk1.append(l1.val)
            l1 = l1.next
        while l2:
            stk2.append(l2.val)
            l2 = l2.next

        prev, head = None, None
        sum = 0
        while stk1 or stk2:
            sum /= 10
            if stk1:
                sum += stk1.pop()
            if stk2:
                sum += stk2.pop()

            head = ListNode(sum % 10)
            head.next = prev
            prev = head

        if sum >= 10:
            head = ListNode(sum / 10)
            head.next = prev

        return head