LeetCode 2, Add Two Numbers

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.（给定两个链表代表两个非负数，数字是反序存放的，并且每一个节点只存放一位，将这两个数加起来以链表形式并返回）

例如：

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* l=l1;        ListNode* m=l2;        int a=0;        if(l1==NULL)                return l2;        else if(l2==NULL)                return l1;        else{                while(l1->next!=NULL&&l2->next!=NULL)                {                        a+=l1->val+l2->val;                        if(a>9)                        {                            l1->val=a%10;                            l2->val=a%10;                            a=1;                        }                        else                        {                            l1->val=a;                            l2->val=a;                            a=0;                        }                        l1=l1->next;                        l2=l2->next;                }                if(l1->next==NULL&&l2->next==NULL)                {                        if(l1->val+l2->val+a>9)                                {                                        l1->val=(l1->val+l2->val+a)%10;                                        l1->next=new ListNode(1);                                }                        else l1->val=l1->val+l2->val+a;                        return l;                }                else if(l1->next==NULL)                {                        if(l1->val+l2->val+a>9)                        {                                l2->val=(l1->val+l2->val+a)%10;                                while(l2->next!=NULL)                                {                                        l2=l2->next;                                        if(l2->val==9)                                                l2->val=0;                                        else                                        {                                                l2->val+=1;                                                break;                                        }                                }                                if(l2->next==NULL&&l2->val==0)                                {                                     l2->next=new ListNode(1);                                   }                        }                        else l2->val+=l1->val+a;                        return m;                }                else                {                        if(l1->val+l2->val+a>9)                        {                                l1->val=(l1->val+l2->val+a)%10;                                while(l1->next!=NULL)                                {                                        l1=l1->next;                                        if(l1->val==9)                                                l1->val=0;                                        else                                        {                                                l1->val+=1;                                                break;                                        }                                }                                if(l1->next==NULL&&l1->val==0)                                {                                     l1->next=new ListNode(1);                                   }                        }                        else l1->val+=l2->val+a;                        return l;                }        }    }};