2：两个数字相加（add two numbers）

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.

    public static ListNode addTwoNumbers(ListNode l1,ListNode l2){
    /*
    * 本题的情况分别有；
    * （1）一个链表比另一个链表长：l1：0->1,l2：0->1->2
    * （2）一个链表为空：l1=null,l2：0->1
    * (3)有额外的进位：l1:9->9,l2: 1
    * (4)正常情况：两个链表一样长
    * */
    ListNode l3=new ListNode(0);
    ListNode head=l3;
    int carry=0;
    while (l1!=null||l2!=null){
        int x=(l1!=null)?l1.val:0;
        int y=(l2!=null)?l2.val:0;
        int sum=x+y+carry;
        carry=sum/10;
        l3.next=new ListNode(sum%10);
        l3=l3.next;
        if (l1!=null) l1=l1.next;
        if (l2!=null) l2=l2.next;
    }
    if (carry>0){
        l3.next=new ListNode(carry);
    }
    return head.next;
}