2. 两数相加

/**
 * 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) {
        if (l1 == NULL) return l2;
        if (l2 == NULL) return l1;
        if (l1 == NULL && l2 == NULL) return NULL;
        ListNode* dummyHead = new ListNode(0);
        ListNode* p = l1, *q = l2;
        ListNode* current = dummyHead;
        int carry = 0;
        while (p != NULL || q != NULL)  
        {
            int x = (p != NULL) ? p->val : 0;
            int y = (q != NULL) ? q->val : 0;
            int sum = carry + x + y;
            carry = sum / 10;
            current->next = new ListNode(sum % 10);
            current = current->next;
            if (p != NULL) p = p->next;
            if (q != NULL) q = q->next;
        }
        if (carry > 0 )
        {
            current->next = new ListNode(carry);
        }
        return  dummyHead->next;
        
    }
};