2. 两数相加

解：

/**
 * 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 head = new ListNode();
        ListNode curr = head;
        int x = 0, v1 = 0, v2 = 0, v = 0;
        while(l1 != null || l2 != null){
            v1 = (l1 == null) ? 0: l1.val;
            v2 = (l2 == null) ? 0: l2.val;
            v = (v1 + v2 + x) % 10;
            x = (v1 + v2 + x) / 10;
            curr.val = v;
            l1 = (l1 == null) ? null: l1.next;
            l2 = (l2 == null) ? null: l2.next;
            if(l1 == null && l2 == null && x == 0){
                return head;
            }
            ListNode after = new ListNode(1);
            curr.next = after;
            curr = curr.next;
        }
        return head;
    }
}