求求了，救救孩子吧

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <queue>
#define ll long long
#define inf 0x3f
using namespace std;
const int maxn = 100005;
int n, k, tot, head[maxn], w[maxn << 1], v[maxn << 1], nxt[maxn << 1], dis[maxn], pre[maxn], f[maxn], vis[maxn];
queue<int> q;
void add(int x, int y) {
	v[++tot] = y;
	w[tot] = 1;
	nxt[tot] = head[x];
	head[x] = tot;
}
int bfs(int s) {
	memset(dis, inf, sizeof(dis));
	q.push(s);
	dis[s] = pre[s] = 0;
	while (!q.empty()) {
		int x = q.front();
		q.pop();
		for (int i = head[x]; i; i = nxt[i]) {
			if (dis[v[i]] >= dis[x] + w[i]) {
				dis[v[i]] = dis[x] + w[i];
				pre[v[i]] = i;
				q.push(v[i]);
			}
		}
	}
	int point, ans = -inf;
	for (int i = 1; i <= n; i++) {
		if (ans < dis[i]) {
			ans = dis[i];
			point = i;
		}
	}
	return point;
}
namespace k1 {
	int diameter() {
		return dis[bfs(bfs(1))];
	}
	void solve() {
		int D = diameter();
		cout << 2 * (n - 1) - D + 1 << endl;
	}
}
namespace k2 {
	int point = bfs(bfs(1)), ans = -inf;
	int diameter() {
		return dis[point];
	}
	void change() {
		for (; pre[point]; point = v[pre[point] ^ 1]) {
			w[pre[point]] = w[pre[point] ^ 1] = -1;
		}
	}
	void dp(int u) {
		vis[u] = 1;
		for (int i = head[u]; i; i = nxt[i]) {
			if (!vis[v[i]]) {
				dp(v[i]);
				ans = max(ans, f[u] + f[v[i]] + w[i]);
				f[u] = max(f[u], f[v[i]] + w[i]);
			}
		}
	}
	void solve() {
		int D = diameter();
		change();
		dp(1);
		cout << 2 * n - D - ans << endl;
	}
}
int main() {
	cin >> n >> k;
	tot = 1;
	for (int i = 1; i < n; i++) {
		int a, b;
		cin >> a >> b;
		add(a, b), add(b, a);
	}
	if (k == 1) {
		k1::solve();
	} else {
		k2::solve();
	}
	system("pause");
	return 0;
}