记录编号	576993	评测结果	AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
题目名称	[APIO 2010]巡逻	最终得分	100
用户昵称	HeSn	是否通过	通过
代码语言	C++	运行时间	1.684 s
提交时间	2022-10-19 22:19:39	内存使用	11.16 MiB

#include<bits/stdc++.h>
using namespace std;
const int MAXN = 100010;
int n, k, ans, maxn, rt, vis[MAXN], fa[MAXN], f[MAXN], flg;
vector<int>cd[MAXN], w[MAXN];
void dfs(int x, int s, int fath) {
//	cout << x << ' ' << s << endl;;
	if(s > maxn) {
		maxn = s;
		rt = x;
	}
	for(int i = 0; i < cd[x].size(); i ++) {
		int u = cd[x][i];
		if(u == fath) {
			continue;
		}
		if(flg) {
			fa[u] = x;
		}
		dfs(u, s + 1, x);
	}
}
void dp(int x, int fath) {
	for(int i = 0; i < cd[x].size(); i ++) {
		int u = cd[x][i];
		if(u == fath) {
			continue;
		}
		dp(u, x);
		ans = max(ans, f[x] + f[u] + w[x][i]);
		f[x] = max(f[x], f[u] + w[x][i]);
	}
}
signed main() {
    freopen("xunluo.in", "r", stdin);
    freopen("xunluo.out", "w", stdout);
	cin >> n >> k;
	for(int i = 1; i < n; i ++) {
		int x, y;
		cin >> x >> y;
		cd[x].push_back(y);
		cd[y].push_back(x);
		w[x].push_back(1);
		w[y].push_back(1);
	}
	dfs(1, 0, 0);
	flg = 1;
//	cout << rt << endl;
	maxn = 0;
	dfs(rt, 0, 0);
//	cout << rt << endl;
	if(k == 1) {
		cout << n * 2 - maxn - 1;
		return 0;
	}
//	cout << maxn << endl;
	ans = n * 2 - maxn;
	int x = rt;
//	for(int i = 1; i <= n; i ++) {
//		cout << fa[i] << ' ';
//	}
	while(fa[x]) {
//		cout << x << endl;
		for(int i = 0; i < cd[fa[x]].size(); i ++) {
			int u = cd[fa[x]][i];
			if(u == x) {
				w[fa[x]][i] = -1;
				break;
			}
		}
		for(int i = 0; i < cd[x].size(); i ++) {
			int u = cd[x][i];
			if(u == fa[x]) {
				w[x][i] = -1;
				break;
			}
		}
		x = fa[x];
	}
	x = ans;
	ans = 0;
	dp(1, 0);
//	cout << ans << ' ' << x << endl;
	cout << x - ans;
	return 0;
}