This problem was recently asked by Microsoft:

You are given two 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.

```plaintext
Example:

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

Here is the function signature as a starting point (in Python):

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

class Solution:
  def addTwoNumbers(self, l1, l2, c = 0):
    # Fill this in.

l1 = ListNode(2)
l1.next = ListNode(4)
l1.next.next = ListNode(3)

l2 = ListNode(5)
l2.next = ListNode(6)
l2.next.next = ListNode(4)

result = Solution().addTwoNumbers(l1, l2)
while result:
  print result.val,
  result = result.next
# 7 0 8
```