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;
    }
};