两数相加

两个链表表示非负整数，链表逆序存储了数据的每一位，求两个数之和。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(-1);
        ListNode end = dummy;
        int c = 0;
        while(l1 != null && l2 != null) {
            int s = l1.val + l2.val + c;
            c = s / 10;
            ListNode node = new ListNode(s % 10);
            end = end.next = node;
            l1 = l1.next;
            l2 = l2.next;
        }
        while(l1 != null) {
            int s = l1.val + c;
            c = s / 10;
            ListNode node = new ListNode(s % 10);
            end = end.next = node;
            l1 = l1.next;
        }
        while(l2 != null) {
            int s = l2.val + c;
            c = s / 10;
            ListNode node = new ListNode(s % 10);
            end = end.next = node;
            l2 = l2.next;
        }
        if(c == 1) {
            ListNode node = new ListNode(c);
            end = end.next = node;
        }
        return dummy.next;
    }
}