题意：两个长度为[1, 100]的大数，分别倒序存储（个位在链表头）在两个链表中，计算两个数的和，并倒序存储在一个新链表，返回链表表头。数据中不存在前导零。
题解：模拟大数相加，注意维护进位carry即可
代码
/**
* 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, tail;
head = tail = null;
int carry = 0;
while(l1 != null || l2 != null) {
int val1 = 0, val2 = 0;
if(l1 != null) val1 = l1.val;
if(l2 != null) val2 = l2.val;
int sum = val1 + val2 + carry;
if(sum >= 10) {carry = 1; sum %= 10;}
else carry = 0;
if(head == null) {
head = tail = new ListNode(sum, null);
} else {
tail.next = new ListNode(sum, null);
tail = tail.next;
}
if(carry == 1) tail.next = new ListNode(carry, null);
if(l1 != null) l1 = l1.next;
if(l2 != null) l2 = l2.next;
}
return head;
}
}