leetcode 21

合并两个有序数列。属于基础的数据结构问题，核心在于对链表的操作。

代码如下：

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode(int x) : val(x), next(NULL) {}
  * };
  */
 class Solution {
 public:
     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
         ListNode* head1 = NULL;
         ListNode* head2 = NULL;
         ListNode* tmp = NULL;
         if(l1 == NULL)
         {
             return l2;
         }
         else if(l2 == NULL)
         {
             return l1;
         }
         else if(l1->val > l2->val)
         {
             tmp = l2;
             head1 = l1;
             head2 = l2->next;
         }
         else
         {
             tmp = l1;
             head1 = l1->next;
             head2 = l2;
         }
         ListNode* node = tmp;
         while(true)
         {
             if(head1 == NULL)
             {
                 node->next = head2;
                 break;
             }
             if(head2 == NULL)
             {
                 node->next = head1;
                 break;
             }
             if(head1->val > head2->val)
             {
                 node->next = head2;
                 head2 = head2->next;
             }
             else
             {
                 node->next = head1;
                 head1 = head1->next;
             }
             node = node->next;
         }
         return tmp;
     }
 };