Add Two Numbers

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) {}
 };

Code:

 ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        size_t lengthL1 = 0;
        size_t lengthL2 = 0;
        ListNode *result = NULL;
        ListNode *pL1 = l1;
        ListNode *pL2 = l2;
        while (pL1)
        {
            lengthL1++;
            pL1 = pL1->next;
        }
        while (pL2)
        {
            lengthL2++;
            pL2 = pL2->next;
        }
        
        ListNode *longList = (lengthL1 >= lengthL2)?l1:l2;
        ListNode *shortList = (lengthL1 < lengthL2)?l1:l2;
        
        //设置进位标志
        int flag = 0;
        int x;
        ListNode *p = NULL;
        while (shortList)
        {
            x = flag + longList->val + shortList->val;
            flag = x/10;
            if (result==NULL)
            {
                result = new ListNode ( x%10 );
                p = result;
            }
            else
            {
                p->next = new ListNode (x%10);
                p = p->next;
            }
            shortList = shortList->next;
            longList = longList->next;
        }
         while (longList)
        {
            if (flag == 0)
            {
                   p->next = longList; 
                   return result;
            }
            else
            {
                x = flag + longList->val;
                flag = x/10;
                p->next = new ListNode (x%10);
                p = p->next;
                longList = longList->next;
             }
        }
            if (flag==1)
            {
                p->next = new ListNode (1);
            }
            return result;
    }