LeetCode之“链表”：Add Two Numbers

　　题目链接

　　题目要求：

　　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

　　这道题难度不大，具体程序如下：

 /**
  * 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)
             return l2;
         else if(!l2)
             return l1;

         ListNode *dummy = new ListNode();
         ListNode *head = nullptr, *start = dummy;
         int carry = ;
         while(l1 || l2)
         {
             if(l1 && l2)
             {
                 int sum = l1->val + l2->val + carry;
                 l1->val = sum % ;
                 carry = sum / ;
                 start->next = l1;
                 l1 = l1->next;
                 l2 = l2->next;
             }
             else if(l1)
             {
                 int sum = l1->val + carry;
                 l1->val = sum % ;
                 carry = sum / ;
                 start->next = l1;
                 l1 = l1->next;
             }
             else
             {
                 int sum = l2->val + carry;
                 l2->val = sum % ;
                 carry = sum / ;
                 start->next = l2;
                 l2 = l2->next;
             }

             start = start->next;
         }

         if(carry)
         {
             ListNode *node = new ListNode();
             start->next = node;
         }

         head = dummy->next;
         delete dummy;
         dummy = nullptr;

         return head;
     }
 };