21. Merge Two Sorted Lists 合并两个排序的列表

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

如题，题意大致为将两个排好序的列表重新放入一个排好序的新列表中，新列表必须有两个列表的节点组成。

通过比较两个列表中每个节点的值的大小，采用递归思想，值较小则鸡血

class Solution {public:    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {        //递归排序        if(l1 == NULL) return l2;        if(l2 == NULL) return l1;        ListNode *temp = NULL;        if(l1->val < l2->val)        {            temp = l1;            temp->next = mergeTwoLists(l1->next,l2);        }        else        {            temp = l2;            temp->next = mergeTwoLists(l1,l2->next);        }        return temp;    }};