Leetcode--Add Two Numbers

Problem Description:

You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        ListNode *p=l1, *q=l2;
        ListNode *head=NULL,*cur=NULL;
        if(!p&&!q)
            return head;
        int flag=0;
        while(p&&q)
        {
            int value=(p->val+q->val+flag)%10;
            flag=(p->val+q->val+flag)/10;
            ListNode *temp=new ListNode(value);
            if(!cur)
            {
                cur=temp;
                head=cur;
            }
            else
            {
                cur->next=temp;
                cur=temp;
            }
            p=p->next;
            q=q->next;
        }
        while(p)
        {
            int value=(p->val+flag)%10;
            flag=(p->val+flag)/10;
            p->val=value;
            cur->next=p;
            cur=p;
            p=p->next;
        }
        while(q)
        {
            int value=(q->val+flag)%10;
            flag=(q->val+flag)/10;
            q->val=value;
            cur->next=q;
            cur=q;
            q=q->next;
        }
        if(flag==1)
        {
            ListNode *temp=new ListNode(1);
            cur->next=temp;
        }
        
        return head;
    }
};