#include <cstring>
#include <iostream>
#include <algorithm>
#define x first
#define y second
using namespace std;
typedef pair<int, int> PII;

const int N = 2e5+10;
int e[N],ne[N],he[N],idx;
int dis[N];
int ans;
bool st[N];
int n,d;
void add(int a,int b){
	e[idx] = b;
	ne[idx] = he[a];
	he[a] = idx++; 
}
void dfs(int u){
	for(int i = he[u];i;i = ne[i]){
		int v = e[i];
		if(dis[v]) continue;
		dis[v] = dis[u]+1;
		dfs(v);
	}
	return;
}
int main()
{
	cin>>n>>d;
	for(int i=1;i<n;i++){
		int a,b;
		scanf("%d%d",&a,&b);
		add(a,b);
		add(b,a);
	}
	dis[1] = 1;
	dfs(1);
	for(int i =2;i<=n;i++){
		if(dis[i]-1<=d)ans++;
	}
	cout<<ans;

    return 0;
}

上看代码能ac，但是过不了样例 下面代码能过样例 但是答案错误 我不理解

#include <cstring>
#include <iostream>
#include <algorithm>
#define x first
#define y second
using namespace std;
typedef pair<int, int> PII;

const int N = 2e5+10;
int e[N],ne[N],he[N],idx;
int dis[N];
int ans;
bool st[N];
int n,d;
void add(int a,int b){
	e[idx] = b;
	ne[idx] = he[a];
	he[a] = idx++; 
}
void dfs(int u){
	for(int i = he[u];i;i = ne[i]){
		int v = e[i];
		if(dis[v]) continue;
		dis[v] = dis[u]+1;
		dfs(v);
	}
	return;
}
int main()
{
	cin>>n>>d;
	for(int i=1;i<n;i++){
		int a,b;
		scanf("%d%d",&a,&b);
		add(a,b);
		add(b,a);
	}
	//dis[1] = 1;
	dfs(1);
	for(int i =2;i<=n;i++){
		if(dis[i]<d)ans++;
	}
	cout<<ans;

    return 0;
}