leetcode Add Two Numbers

题目：

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

代码：

/*直接取对应位置的两个数字相加，进位单独记录。需要注意的是运算结束时，结尾可能会    有进位。总体时间为O（N）*/    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        int carry = 0;//表示进位        ListNode result = new ListNode(0);        ListNode ptr = result;//指针        while (l1 != null || l2 != null) {            int val1 = 0;            if (l1 != null) {                val1 = l1.val;                l1 = l1.next;            }            int val2 = 0;            if (l2 != null) {                val2 = l2.val;                l2 = l2.next;            }            int temp = val1 + val2 + carry;            ptr.next = new ListNode(temp % 10);            carry = temp / 10;            ptr = ptr.next;        }        if (carry == 1)            ptr.next = new ListNode(1);        return result.next;    }