两个数相加

/**
 * 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) {
        if(l1==null){
            return l2;
        }
        if(l2==null){
            return l1;
        }
        //进位的数字
        int carry=0;
        ListNode head=new ListNode(-1);
        ListNode pre=head;
        while(l1!=null&&l2!=null){
            int number=l1.val+l2.val+carry;
            carry=number/10;
            ListNode node=new ListNode(number%10);
            pre.next=node;
            pre=pre.next;
            l1=l1.next;
            l2=l2.next;
        }
        //对
        while(l1!=null){
            int number=l1.val+carry;
            carry=number/10;
            ListNode node=new ListNode(number%10);
            pre.next=node;
            pre=pre.next;
            l1=l1.next;
        }
        while(l2!=null){
            int number=l2.val+carry;
            carry=number/10;
            ListNode node=new ListNode(number%10);
            pre.next=node;
            pre=pre.next;
            l2=l2.next;
        }
        if(carry!=0){
            ListNode node=new ListNode(carry);
            pre.next=node;
        }
    return head.next;
    }
}