You are given two non-empty linked lists representing two non-negative
integers. 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.

You may assume the two numbers do not contain any leading zero, except
the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 0 -> 8 Explanation:
342 + 465 = 807.

考察大数相加，注意进位，加法是否完成取决于进位，两个链表任意不为空，循环中这几个条件都不满足才退出
这种或的循环一定要注意循环体内有些条件已经不满足

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    int carry=0;
    ListNode head=new ListNode(0);
    ListNode cur=head;
    while(l1!=null || l2!=null || carry>0){
        int sum=carry;
        if(l1!=null){
            sum+=l1.val;
            l1=l1.next;
        }
        if(l2!=null){
            sum+=l2.val;
            l2=l2.next;
        }
        if(sum>=10){
            sum-=10;
            carry=1;
        }else carry=0;
        cur.next=new ListNode(sum);
        cur=cur.next;
    } 
    return head.next;   
}