| # | Time | Username | Problem | Language | Result | Execution time | Memory |
|---|---|---|---|---|---|---|---|
| 1188840 | g4yuhg | Race (IOI11_race) | C++20 | 0 ms | 0 KiB |
#include<bits/stdc++.h>
typedef long long ll;
#define endl '\n'
#define fi first
#define se second
#define pii pair<ll,ll>
#define N 200005
#define LOG 19
#define inf 1e9
using namespace std;
ll n, k, del[N], d[N], ans = inf, child[N];
vector<pii> adj[N];
void cnt_child(ll u, ll parent) {
child[u] = 1;
for (auto v : adj[u]) {
if (v.fi == parent || del[v.fi]) continue;
cnt_child(v.fi, u);
child[u] += child[v.fi];
}
}
ll centroid(ll u, ll parent, ll n) {
for (auto v : adj[u]) {
if (v.fi == parent || del[v.fi]) continue;
if (child[v.fi] > n / 2) {
return centroid(v.fi, u, n);
}
}
return u;
}
map<ll, ll> mp;
void dfs(ll u, ll parent, ll dep) {
if (mp.find(k - d[u]) != mp.end()) {
ans = min(ans, dep + mp[k - d[u]]);
}
if (d[u] == k) ans = min(ans, dep);
for (auto v : adj[u]) {
if (v.fi == parent || del[v.fi]) continue;
d[v.fi] = d[u] + v.se;
if (d[v.fi] > k) continue;
dfs(v.fi, u, dep + 1);
}
}
void dfs2(ll u, ll parent, ll dep) {
if (mp.find(d[u]) == mp.end()) {
mp[d[u]] = dep;
}
else {
mp[d[u]] = min(mp[d[u]], dep);
}
for (auto v : adj[u]) {
if (v.fi == parent || del[v.fi]) continue;
d[v.fi] = d[u] + v.se;
if (d[v.fi] > k) continue;
dfs2(v.fi, u, dep + 1);
}
}
void gq(ll x) {
mp.clear();
d[x] = 0;
for (auto v : adj[x]) {
if (del[v.fi]) continue;
d[v.fi] = d[x] + v.se;
dfs(v.fi, x, 1);
dfs2(v.fi, x, 1);
}
}
void solve(ll u) {
cnt_child(u, u);
ll x = centroid(u, u, child[u]);
gq(x);
del[x] = 1;
for (auto v : adj[x]) {
if (del[v.fi]) continue;
solve(v.fi);
}
}
signed main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n >> k;
for (int i = 1; i <= n - 1; i ++) {
ll u, v, w;
cin >> u >> v >> w;
u ++ ;
v ++ ;
adj[u].push_back({v, w});
adj[v].push_back({u, w});
}
solve(1);
if (ans == inf) {
ans = -1;
}
cout << ans;
return 0;
}