【问】关于Splay

		struct Splay_Node{
			ll sum;
			int siz;
			Splay_Node* ls;
			Splay_Node* rs;
			Splay_Node* fa;
			Splay_Node(ll _sum) : siz(1), ls(NULL), rs(NULL), fa(NULL) { sum=_sum; }
		};
		Splay_Node* kth(Splay_Node* rt,int k){
			//if(rt==NULL||k>rt->siz) throw runtime_error("You can't find kth in the empty tree!");
			int lsiz=0;
			if(rt->ls==NULL) lsiz=0;
			else lsiz=rt->ls->siz;
			if(k==lsiz+1) return rt;
			if(k<=lsiz) return kth(rt->ls,k);
			return kth(rt->rs,k-lsiz-1);
		}