[LeetCode]--2. Add Two Numbers

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

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode ln1, head;
        int flag = 0;
        head = ln1 = new ListNode(0);
        while (l1 != null || l2 != null) {
            if (l1 != null && l2 != null) {
                if (l1.val + l2.val + flag >= 10) {
                    ln1.val = l1.val + l2.val + flag - 10;
                    flag = 1;
                } else {
                    ln1.val = l1.val + l2.val + flag;
                    flag = 0;
                }
                l1 = l1.next;
                l2 = l2.next;
            } else if (l1 == null) {
                if (l2.val + flag >= 10) {
                    ln1.val = l2.val + flag - 10;
                    flag = 1;
                } else {
                    ln1.val = l2.val + flag;
                    flag = 0;
                }
                l2 = l2.next;
            } else if (l2 == null) {
                if (l1.val + flag >= 10) {
                    ln1.val = l1.val + flag - 10;
                    flag = 1;
                } else {
                    ln1.val = l1.val + flag;
                    flag = 0;
                }
                l1 = l1.next;
            }
            if (l1 != null || l2 != null) 
                ln1.next = new ListNode(0);
            if (flag == 1) {
                ln1.next = new ListNode(0);
                ln1.next.val = 1;
            }
            ln1 = ln1.next; 
        }
        return head;
    }

这是我写的，虽然通过了但是我觉得还可以提高，点了一下看到了一个别人写程序，亮瞎我眼了。

/**
     * 很简便的一个写法，至少甩我两条街
     * @param l1
     * @param l2
     * @return
     */
    public ListNode addTwoNumbers2(ListNode l1, ListNode l2) {
        ListNode dummyHead = new ListNode(0);
        ListNode p = l1, q = l2, curr = dummyHead;
        int carry = 0;
        while (p != null || q != null) {
            int x = (p != null) ? p.val : 0;
            int y = (q != null) ? q.val : 0;
            int sum = carry + x + y;
            carry = sum / 10;
            curr.next = new ListNode(sum % 10);
            curr = curr.next;
            if (p != null) p = p.next;
            if (q != null) q = q.next;
        }
        if (carry > 0) {
            curr.next = new ListNode(carry);
        }
        return dummyHead.next;
    }