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OI-codes/Luogu/P4149/P4149.cpp

117 lines
2.2 KiB
C++

#include <iostream>
#include <limits>
#include <set>
#include <vector>
using std::cin;
using std::cout;
const char endl = '\n';
const int N = 2e5 + 5;
int n, k, ans = std::numeric_limits<int>::max();
int root, siz[N], max[N];
int cnt;
std::pair<int, int> dist[N];
bool vis[N];
std::vector<std::pair<int, int>> g[N];
void find(int u, int fa, int tot) {
siz[u] = 1;
max[u] = 0;
for (auto e : g[u]) {
int v = e.first,
w = e.second;
if (v == fa || vis[v]) continue;
find(v, u, tot);
siz[u] += siz[v];
max[u] = std::max(max[u], siz[v]);
}
max[u] = std::max(max[u], tot - siz[u]);
if (max[u] < max[root]) root = u;
}
void dis(int u, int fa, int w, int l) {
if (w > k || l > ans) return;
dist[++cnt] = std::make_pair(w, l);
for (auto e : g[u]) {
int _v = e.first,
_w = e.second;
if (_v == fa || vis[_v]) continue;
dis(_v, u, w + _w, l + 1);
}
}
void calc(int u) {
std::set<std::pair<int, int>> set;
set.insert(std::make_pair(0, 0));
for (auto e : g[u]) {
int v = e.first,
w = e.second;
if (vis[v]) continue;
cnt = 0;
dis(v, u, w, 1);
for (int i = 1; i <= cnt; i++) {
auto it = set.lower_bound(std::make_pair(k - dist[i].first, 0));
if (it != set.end() && it->first + dist[i].first == k) {
ans = std::min(ans, it->second + dist[i].second);
}
}
for (int i = 1; i <= cnt; i++) set.insert(dist[i]);
}
}
void solve(int u) {
vis[u] = true;
calc(u);
for (auto e : g[u]) {
int v = e.first,
w = e.second;
if (vis[v]) continue;
max[root = 0] = std::numeric_limits<int>::max();
find(v, 0, siz[v]);
solve(root);
}
}
int main() {
std::ios::sync_with_stdio(false);
cin >> n >> k;
for (int i = 1, u, v, w; i < n; i++) {
cin >> u >> v >> w;
g[u + 1].push_back(std::make_pair(v + 1, w));
g[v + 1].push_back(std::make_pair(u + 1, w));
}
max[root = 0] = n;
find(1, 0, n);
solve(root);
cout << (ans == std::numeric_limits<int>::max() ? -1 : ans) << endl;
return 0;
}