# Two Lists Sum Advanced </i>

## Question

Given two numbers represented by two linked lists, write a function that returns sum list.
The sum list is linked list representation of addition of two input numbers.

Example

Input:
First  List: 5->6->3  // represents number 563
Second List: 8->4->2  // represents number 842
Output
Resultant list: 1->4->0->5  // represents number 1405

Challenge

Not allowed to modify the lists.
Not allowed to use explicit extra space.