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) {
        //维护一个新的链表，分别用两个指针指向链表,应当注意头结点和后面的节点操作不一样
        if(l1==NULL)
        return l2;
        if(l2==NULL)
        return l1;
        ListNode*head= NULL;//头指针
        ListNode*cur= NULL;//当前操作的点
        ListNode*p =l1;
        ListNode*q =l2;
      
        while(p&&q)
        {if (head == NULL)
           {if (p->val<q->val)
              {
              head = cur= p;
              p= p->next;
              head->next = NULL;
              }
             
            else
            {
              head = cur = q;
              q= q->next;
              head->next = NULL;
               
            }
           }
       
        else
        {
          if (p->val<q->val)
              {
              cur->next= p;
              cur= cur->next ;
              p= p->next;
              cur->next = NULL;
              } 
         else
           {
            cur->next= q;
            cur= cur->next ;
            q= q->next;
            cur->next = NULL;
           }
        }
    }
       
    if(p!= NULL)
    cur->next= p;
    else if(q!= NULL)
    cur->next = q;
    return head;
    }
};