LeetCode #2 Add Two Numbers (M)

[Problem]

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

[Analysis]

这题的思路比较简单，只需要把两个ListNode相加可能出现的四种情况列清楚：

1. L1 != null && L2 != null

2. L1 != null && L2 == null

3. L1 ==? null && L2 != null

4. ?L1 == L2 == null && carry == 1

[Solution]

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode currentNode = new ListNode(0);
        return addTwoNumbers(l1, l2, currentNode, 0).next;
    }

    public ListNode addTwoNumbers(ListNode l1, ListNode l2, ListNode currentNode, int carry) {
        int val1 = 0;
        int val2 = 0;

        if (l1 != null) {
            val1 = l1.val;
            l1 = l1.next;

            if (l2 != null) {
                val2 = l2.val;
                l2 = l2.next;
            }
        } else {
            if (l2 != null) {
                val2 = l2.val;
                l2 = l2.next;
            } else {
                if (carry != 0) {
                    currentNode.next = new ListNode(carry);
                }

                return currentNode;
            }
        }

        int val = val1 + val2 + carry;
        ListNode newNode = new ListNode(val % 10);
        currentNode.next = addTwoNumbers(l1, l2, newNode, val/10);        

        return currentNode;
    }
}