2. Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each node contains a single digit. Add the two numbers and return the sum as a linked list in the same reverse-digit format. For example, l1 = [2,4,3] represents the number 342, and l2 = [5,6,4] represents the number 465. Their sum is 342 + 465 = 807, which is represented as [7,0,8]. You may assume the two numbers do not contain any leading zeros, except the number 0 itself.