Add Two Numbers(leetcode)

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
#define null 0
struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {
    struct ListNode* p;
    struct ListNode* q;
    p=l1;
    q=l2;
    int flag=0;
    int sum;
    for(p=l1,q=l2;p->next!=null && q->next!=null;p=p->next,q=q->next){
        p->val += q->val;
        p->val +=flag;
        flag = p->val / 10;
        if(flag){
            p->val %= 10;
        }
    }
    p->val = p->val + q->val + flag;
    flag = p->val / 10;
    p->val %= 10;
    if(q->next){
        p->next=q->next;
    }
    while(p->next){
        p=p->next;
        p->val += flag;
        flag = p->val / 10;
        if(flag){
            p->val %= 10;
        }
    }
    if(flag){
        struct ListNode* temp;
        temp=(struct ListNode*)malloc(sizeof(struct ListNode));
        temp->val = flag;
        temp->next=null;
        p->next=temp;
    }
    return l1;
}