[LeetCode] 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) {
        ListNode *head = NULL,*p=NULL,*temp;
        if(l1 != NULL || l2 !=NULL){
            if(l1 != NULL && l2 !=NULL){
               if(l1->val < l2->val){
                   head = l1;
                   l1= l1->next;
               }
               else{
                   head = l2;
                   l2 = l2->next;
               }
            }
            else if(l1 != NULL){
                head = l1;
                l1= l1->next;
            }
            else{
                head = l2;
                l2 = l2->next;
            }
        }//end if
        
            p = head;
        while(l1 != NULL && l2 != NULL){
            if(l1->val<l2->val){
                temp = l1;
                l1 = l1->next;
            }
            else{
                temp = l2;
                l2 = l2->next;
            }
            p->next = temp;
            p = p->next;
        }//end while
        
        while(l1 != NULL){
            p->next = l1;
            l1 = l1->next;
            p = p->next;
        }//end while
        
        while(l2 != NULL){
            p->next = l2;
            l2 = l2->next;
            p = p->next;
        }//end while
         if(p)
            p->next = NULL;
          return head;  
        
           
    }
};