LeetCode 习题 2. 两数相加 给定两个非空链表来表示两个非负整数。位数按照逆序方式存储，它们的每个节点只存储单个数字。将两数相加返回一个新的链表。 你可以假设除了数字 0 之外，这两个数

系统题目给出的l1, l2为不带头结点的链表。我开始用的是带头结点的链表 。。。代码还需改进。。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {
    struct ListNode *p, *q, *s, *r, *d;
    int a, b;
    s = (struct ListNode*) malloc(sizeof(struct ListNode));
    s->next = NULL;
    r = s;
    s->val = l1->val + l2->val;
    p = l1->next; 
    q = l2->next;
    while(q!=NULL&&p!=NULL){
        d = (struct ListNode*) malloc(sizeof(struct ListNode));
        d->next = NULL;
        d->val = q->val + p->val;
        r->next = d;
        r = r->next;
        p = p->next;
        q = q->next;
    }
    if(p!= NULL)
        r->next = p;
    if(q!= NULL)
        r->next = q;
    r = s;
    while(r!= NULL){
        if(r->val>9){
            a = r->val / 10;
            b = r->val % 10;
            r->val = b;
            if(r->next!=NULL){
                r->next->val = r->next->val + a;
            }else{
                d = (struct ListNode*) malloc(sizeof(struct ListNode));
                d->next = NULL;
                d->val = a;
                r->next = d;
            }
        }
        r = r->next;
    }
    return s;
}