思路：广度优先搜索
题目： https://www.luogu.com.cn/problem/P5908

#include <iostream>
#include <vector>
#include<map>
#include <string>
#include <algorithm>
#include <cmath>
#include <string>
#include <cstring>
#include <iomanip>
#include<stack>
#include<queue>
#include<set>
using namespace std;
//众所周知，set能有序地维护同一类型的元素，但相同的元素只能出现一次。
const int N = 2e5 + 5;
queue<int> que;
int h[N], e[N], ne[N], idx=0;
bool st[N];
//int cnt_out[N];
int n,maxx,ans=-1;
int  d[N];
void add(int u, int v) {
    e[idx] = v;
    //w[idx] = val;
    ne[idx] = h[u];
    h[u] = idx++;
}
void bfs() {
    que.push(1);
    //d[1] = 0;
    while (que.size())
    {
        int head = que.front();
        que.pop();
        if (d[head] > maxx) return;
        ans += 1;
        for (int i = h[head]; i != -1; i = ne[i]) {
            int t = e[i];
            if (!d[t] && t!=1) {
                d[t] = d[head] + 1;
                que.push(t);
            }
        }
    }
}

int main() {
    memset(h, -1, sizeof h);
    cin >> n >> maxx;
    for (int i = 1; i < n; i++) {
        int u, v;
        cin >> u >> v;
        add(u, v), add(v, u);
    }
    bfs();
    cout << ans << endl;
}