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) {
        if(l1==NULL&&l2==NULL)
            return NULL;
        if(l1==NULL)
            return l2;
        if(l2==NULL)
            return l1;
        ListNode* head;
        if(l1->val<=l2->val)
        {
            head=l1;
            l1=l1->next;
        }
        else
        {
            head=l2;
            l2=l2->next;
        }
        ListNode* p=head; 
        while(l1&&l2)
        {
            ListNode* k=(l1->val<=l2->val)?l1:l2;
            p->next=k;
            p=p->next;
            if(k==l1)
                l1=l1->next;
            else
                l2=l2->next;
        }
        if(l1)
            p->next=l1;
        if(l2)
            p->next=l2;
        return head;
    }
};

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