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;
}
};