LeetCode Merge Two Sorted Lists

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        ListNode head(0);
        ListNode *p=&head;
        while(l1!=NULL&&l2!=NULL)
        {
            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;
    }
};