好久沒做題，邊界case沒有很注意，看來還是需要多做一做。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
   public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
		ListNode result = new ListNode(0);
		ListNode currentNode = result;
		ListNode pre =  null;
		int carry = 0;
        while(l1 != null && l2 != null){
        	currentNode.val = (l1.val + l2.val + carry) % 10;
        	carry =  (l1.val + l2.val + carry) / 10;
        	pre = currentNode;
        	currentNode = new ListNode(0);
        	pre.next = currentNode;
        	l1 = l1.next;
        	l2 = l2.next;
        }
        currentNode = addWithCarry(l1, l2, carry);
        pre.next = currentNode;
        return result;
        
        
    }
	public ListNode addWithCarry(ListNode l1, ListNode l2, int carry){
		if (l1 == null && l2 == null) {
			if (carry != 0) {
				ListNode cur = new ListNode(0);
				cur.val = carry;
				return cur;
			}
			return null;
		}
		if (l1 == null) {
			return addWithCarry(l2, l1, carry);
		}
		ListNode result = new ListNode(0);
		ListNode cur = result;
		ListNode pre =  null;
		while (l1 != null) {
			cur.val = (l1.val + carry) % 10;
        	carry =  (l1.val + carry) / 10;
        	pre = cur;
        	cur = new ListNode(carry);
        	pre.next = cur;
        	l1 = l1.next;
		}
		if (pre.next.val == 0) {
			pre.next = null;
		}
		return result;
	}
}