merge-two-sorted-lists

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        if(!l1)return l2;
        if(!l2)return l1;
        ListNode*head=new ListNode(-1);
        ListNode*p=head;
        while(l1&&l2)
        {
            if(l1->val<l2->val)p->next=l1,p=p->next,l1=l1->next;
            else p->next=l2,p=p->next,l2=l2->next;
        }
        if(l1)p->next=l1;
        if(l2)p->next=l2;
        return head->next;
    }
};