Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two 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) 
    {
        if(l1==NULL && l2==NULL)
            return NULL;
        ListNode* head;
        head=GetNum(l1,l2);
        ListNode* p=head;
        while(l1!=NULL || l2!=NULL)
        {
            p->next=GetNum(l1,l2);
            p=p->next;
        }
        return head;
    }
    ListNode* GetNum(ListNode* &l1,ListNode* &l2)
    {
        if(l1==NULL)
        {
            ListNode* p=l2;
            l2=l2->next;
            return p;
        }
        if(l2==NULL)
        {
            ListNode* p=l1;
            l1=l1->next;
            return p;
        }
        if(l1->val<l2->val)
        {
            ListNode* p=l1;
            l1=l1->next;
            return p;
        }
        if(l1->val>=l2->val)
        {
            ListNode* p=l2;
            l2=l2->next;
            return p;
        }
    }
};