2. Add Two Numbers——LeetCode OJ

Difficulty:Medium
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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

给定两个非空单向链表代表两个非负整数。整数中每一位的数字都逆序存储在链表节点中，求出这两个非负整数的和并将结果中的数字以相同的逆序方式存储在一个单项链表中。可以不考虑数字前面的0。

/** * 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;        }        ListNode* tail = new ListNode(0);          ListNode* p = tail;          int carry = 0;         while(l1 != NULL || l2 != NULL)        {              int val1 = 0;              if(l1 != NULL)            {                  val1 = l1->val;                  l1 = l1->next;              }              int val2 = 0;              if(l2 != NULL)            {                  val2 = l2->val;                  l2 = l2->next;              }              int temp = val1 + val2 + carry;              p->next = new ListNode(temp % 10);              carry = temp / 10;              p = p->next;          }             if(carry == 1)          {              p->next = new ListNode(1);            }          return tail->next;      }  };

Runtime: 42 ms