/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2) {

        ListNode* dummy = new ListNode(-1);
        ListNode* cur_p = dummy;     //cur_p为新链表的指针

        while(l1!=0 && l2!=0){
            if(l1->val > l2->val){
                cur_p ->next = l2;
                l2 = l2 ->next;
            }else{
                cur_p ->next= l1;
                l1 = l1 ->next;
            }
            cur_p = cur_p->next;
        }//这样结束的时候会至少有一个l1\l2会指向null

        if(l1!=0)   cur_p->next = l1;
        if(l2!=0)   cur_p->next = l2;

        return dummy->next;

    }  
};