1 /** 2  * Definition for singly-linked list. 3  * struct ListNode { 4  *     int val; 5  *     ListNode *next; 6  *     ListNode(int x) : val(x), next(NULL) {} 7  * }; 8  */ 9  10  void addtolist(int val, int& add, ListNode*&head, ListNode*&pre)11  {12      int sum = val + add;13      if (sum >= 10){14          add = 1;15          sum -= 10;16          17      }18      else {19          add = 0;20      }21      ListNode *node = new ListNode(sum);22      if (!head){23          head = node;24      }25      if (NULL != pre) {26          pre->next = node;27      }28      pre = node;29  }30  31 class Solution {32 public:33     ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {34         if (!l1 && !l2){35             return NULL;36         }37         ListNode *head = NULL;38         ListNode *pre = NULL;39         int add = 0;40         while (l1 != NULL && l2 != NULL){41             addtolist(l1->val + l2->val, add, head, pre);42             l1 = l1->next;43             l2 = l2->next;44         }45         while (l1){46            addtolist(l1->val, add, head, pre);47            l1 = l1->next;48         }49         while (l2){50            addtolist(l2->val, add, head, pre);51             l2 = l2->next;52         }53         if (add){54             addtolist(0, add, head, pre);55         }56         return head;57     }58 };