2.两数相加

创建哨兵节点在返回的链表节点之前，之后采用节点指针遍历，注意判断终止条件，如果有进位也要继续。

/**
 * 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();//哨兵节点
        ListNode current=head;
        int rem=0;
        while(l1!=null||l2!=null||rem!=0){
            if (l1!=null)rem+=l1.val;
            if (l2!=null)rem+=l2.val;
            current.next=new ListNode(rem%10);
            current=current.next;
            rem/=10;
            if(l1!=null)l1=l1.next;
            if(l2!=null)l2=l2.next;
        }
        return head.next;
    }