【Merge Two Sorted Lists】cpp

/**
 * 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 dummy(-1);
            ListNode *p = &dummy;
            while ( l1 && l2 )
            {
                if ( l1->val<l2->val )
                {
                    p->next = l1;
                    l1 = l1->next;
                } 
                else
                {
                    p->next = l2;
                    l2 = l2->next;
                }
                p = p->next;
            }
            p->next = l1 ? l1 : l2;
            return dummy.next;
    }
};