rt，主席树写法，代码如下：

#include<bits/stdc++.h>
#define mid (l+r)/2
using namespace std;
int n,q,head[300001]={0},fa[300001]={0},d[300001]={0},size[300001]={0},dfn[300001]={0},rt[300001]={0},tot=1,opt=0,cnt=0;
struct Edge{
	int to,nxt;
}e[600001];
struct Node{
	int lc,rc;
	long long ans;
}tree[6000001];
void add(int x,int y){
	e[++opt].to=y;
	e[opt].nxt=head[x];
	head[x]=opt;
}
void dfs1(int x,int f){
	size[x]=1;
	for(int i=head[x];i;i=e[i].nxt){
		int v=e[i].to;
		if(v==f) continue;
		tot=max(tot,d[v]=d[x]+1),fa[v]=x;//注释这行后就不MLE了
		dfs1(v,x),size[x]+=size[v];
	}
}
void insert(int &i,int l,int r,int k,int val){
	Node t=tree[i];
	tree[i=++cnt]=t,tree[i].ans+=val;
	if(l==r) return;
	if(k<=mid) insert(tree[i].lc,l,mid,k,val);
	else insert(tree[i].rc,mid+1,r,k,val);
}
void dfs2(int x){
	dfn[x]=++opt,rt[opt]=rt[opt-1];
	insert(rt[opt],1,tot,d[x],size[x]-1);
	for(int i=head[x];i;i=e[i].nxt){
		int v=e[i].to;
		if(v==fa[x]) continue;
		dfs2(v);
	}
}
long long query(int i,int j,int l,int r,int L,int R){
	if(l==L && r==R) return tree[j].ans-tree[i].ans;
	long long ans=0;
	if(L<=mid) ans+=query(tree[i].lc,tree[j].lc,l,mid,L,R);
	if(R>mid) ans+=query(tree[i].rc,tree[j].rc,mid+1,r,L,R);
	return ans;
}
int main(){
	scanf("%d%d",&n,&q);
	for(int i=1;i<=n-1;i++){
		int x,y;
		scanf("%d%d",&x,&y);
		add(x,y),add(y,x);
	}
	d[1]=1,dfs1(1,1);
	opt=0,dfs2(1);
	while(q--){
		int p,k;
		scanf("%d%d",&p,&k);
		if(d[p]==tot) printf("0\n");
		else printf("%lld\n",(long long)min(k,d[p]-1)*(size[p]-1)+query(rt[dfn[p]-1],rt[dfn[p]+size[p]-1],1,tot,d[p]+1,min(tot,d[p]+k)));
	}
	return 0;
}