[Java][LeetCode][Linked List] Add Two Numbers #2
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.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.
Input: l1 = [2,4,3], l2 = [5,6,4]
Explanation: 342 + 465 = 807.
Input: l1 = , l2 = 
Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
- The number of nodes in each linked list is in the range
0 <= Node.val <= 9
- It is guaranteed that the list represents a number that does not have leading zeros.
step1. create a previous node to connect sum of two number and a result node to return.
step2. one by one to sum until one of the list node is null.
step3. check l1 or l2 is not null, this list is longer, and check carry until this node is null.
step4. is carry of sum still have?