leetcode练习：21. Merge Two Sorted Lists

var mergeTwoLists = function(l1, l2) {
   var p = l1;
   var q = l2;
   var k = null;
   var head = null;
  
   if(p == null && q == null)
       return null;
   
   if(p == null )
       return q;
   if(q == null )
       return p;
    
   while(p!=null && q!=null){
       if(head == null) {
           if(p.val <= q.val) {
               head = p;
               p = p.next;
           } else {
               head = q;
               q = q.next;
           }
           k = head;
       } else {
           if(p.val <= q.val) {
               k.next = p;
               p = p.next;
           } else {
               k.next = q;
               q = q.next;
           }     
           k = k.next;
       }
       
   }
    
   if(p!=null) {
       k.next = p;
   }
    
   if(q!=null) {
       k.next = q;
   }
     
   return head;
 
    
};