You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.
l1 = [2,4,3], l2 = [5,6,4]
[7,0,8]
342 + 465 = 807.
nums = [2,7,11,15], target = 9
[0,1]