/**
 * 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 *m = l1;  // 保留原链表
        ListNode *n = l2;

        ListNode *h = new ListNode(-1);
        ListNode *p = h;

        while(m != NULL && n != NULL)
        {
            if(m -> val < n -> val) 
            {
                p -> next = m;
                p = p -> next;
                m = m -> next;
            }
            else
            {
                p -> next = n;
                p = p -> next;
                n = n -> next;
            }
        }

        if(m != NULL) p -> next = m;
        else p -> next = n;

        return h -> next;
    }
};