You are given two non-empty linked lists representing two non-negative integers. 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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

/**
 * 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* p1 = l1;
        ListNode* p2 = l2;
        ListNode* result = new ListNode(0);
        ListNode* n1 = result;
        int val = 0;
        int val1 = 0;
        int val2 = 0;
        int index = 0;

        while(p1!= NULL || p2 !=NULL)
        {
            if(p1 != NULL){
                val1 = p1->val;
            }
            if(p2 != NULL){
                val2 = p2->val;
            }
            val = val1 + val2;
            val += index;
            if(val / 10 >= 1){
                index = 1;
            } else{
                index = 0;
            }

            n1->next = new ListNode(val%10);
            n1 = n1->next;
            if(p1 != NULL){
                p1 = p1->next;
            }
            if(p2 != NULL){
                p2 = p2->next;
            }
            val1 = 0;
            val2 = 0;
        }
        if(index != 0){
            n1->next = new ListNode(1);
        }
        ListNode* res = result->next;
        free(result);
        return res;
    }
};

思路:
分别相加每一位,有进位,val 加1