才学个树和图论就遇“春花怒放”QAQ

#include<bits/stdc++.h>
using namespace std;
struct no{
	int x,r;
}e;
vector <int> tree[100011];
queue <no> s;
bool q[100011];
int n,d,a,b,ans,i;
int main()
{
	scanf("%d %d", &n, &d);
	for(i = 1;i < n;i ++)
	{
		scanf("%d %d",&a,&b);
		tree[a].push_back(b);
		tree[b].push_back(a);
	}
	e.x = 1;
	s.push(e);
	while(!s.empty()){
		no v = s.front();
		s.pop();
		if(v.r>=d)break;
		e.r = v.r + 1;
		for(i = 0;i < tree[v.x].size();i ++)
		{
			if(q[tree[v.x][i]])continue;
			q[tree[v.x][i]] = 1;
			e.x = tree[v.x][i];
			s.push(e);
			ans++;
		}
	}
	printf("%d\n",ans);
	return 0;
}