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public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
ListNode p =l1;
ListNode q = l2;
ListNode head = new ListNode(-1);
ListNode tmp =head;
while(p!=null&&q!=null){
if(p.val<=q.val)
{
tmp.next=p;
p=p.next;
}
else
{
tmp.next=q;
q=q.next;
}
tmp=tmp.next;
}
if(p!=null)
tmp.next=p;
if(q!=null)
tmp.next=q;
//可以将head.next保存下来，然后将head置为null，方便GC
return head.next;
}