You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.
1 <= number of nodes in each list <= 1000 <= Node.val <= 9l1 = [7,2,4,3], l2 = [5,6,4][7,8,0,7]l1 = [2,4,3], l2 = [5,6,4][8,0,7]l1 = [0], l2 = [0][0]