#include <bits/stdc++.h>
using namespace std;

const int kMaxN = 5e4 + 5;

struct Edge {
	int from;
	int to;
	int nxt;
	long long w;
} e[kMaxN * 2];
int tot, head[kMaxN];
void Add(int x, int y, long long w) {
	e[++tot] = (Edge){x, y, head[x], w};
	head[x] = tot;
}

int n, m;
int ans;
multiset<long long> s[kMaxN];

void Dfs(int x, int f, long long k, long long w) {
	s[x].clear();
	for (int i = head[x]; i; i = e[i].nxt) {
		int to = e[i].to;
		if (to == f) {
			continue;
		}
		Dfs(to, x, k, e[i].w);
	}
	long long p = 0;
	multiset<long long>::iterator it = s[x].lower_bound(k);
	multiset<long long>::iterator ed = s[x].end();
	while (ed != it) {
		ed--;
		ans++;
		s[x].erase(ed);
	}
	while (!s[x].empty()) {
		if (s[x].size() == 1) {
			p = max(p, *s[x].begin());
			s[x].erase(s[x].begin());
			break;
		}
		multiset<long long>::iterator pos = s[x].lower_bound(k - *s[x].begin());
    if (pos == s[x].begin()) {
      pos++;
    }
		if (pos == s[x].end()) {
			p = max(p, *s[x].begin());
			s[x].erase(s[x].begin());
			continue;
		}
		ans++;
		s[x].erase(s[x].begin());
		s[x].erase(pos);
	}
	s[f].insert(p + w);
}

bool Check(long long x) {
	ans = 0;
	Dfs(1, 0, x, 0);
	return ans >= m;
}

int main() {
	
	cin >> n >> m;
	for (int i = 1; i <= n - 1; i++) {
		int x, y;
		long long w;
		scanf("%d%d%lld", &x, &y, &w);
		Add(x, y, w);
		Add(y, x, w);
	}
	long long l = 0, r = 1e11;
	while (l + 1 < r) {
		long long mid = (l + r) / 2;
		if (Check(mid)) {
			l = mid;
		}
		else {
			r = mid;
		}
	}
	if (Check(l + 1)) {
		l++;
	}
	printf("%lld\n", l);
	return 0;
}

$Rt$

下载了数据在本机 ($windows-devc++$)过了，但是在洛谷就 $RE$ 了，求大神解释...