#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; using lint = long long; using pint = pair; using plint = pair; struct fast_ios { fast_ios(){ cin.tie(nullptr), ios::sync_with_stdio(false), cout << fixed << setprecision(20); }; } fast_ios_; #define ALL(x) (x).begin(), (x).end() #define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i=i##_begin_;i--) #define REP(i, n) FOR(i,0,n) #define IREP(i, n) IFOR(i,0,n) template void ndarray(vector& vec, const V& val, int len) { vec.assign(len, val); } template void ndarray(vector& vec, const V& val, int len, Args... args) { vec.resize(len), for_each(begin(vec), end(vec), [&](T& v) { ndarray(v, val, args...); }); } template bool chmax(T &m, const T q) { return m < q ? (m = q, true) : false; } template bool chmin(T &m, const T q) { return m > q ? (m = q, true) : false; } int floor_lg(long long x) { return x <= 0 ? -1 : 63 - __builtin_clzll(x); } template pair operator+(const pair &l, const pair &r) { return make_pair(l.first + r.first, l.second + r.second); } template pair operator-(const pair &l, const pair &r) { return make_pair(l.first - r.first, l.second - r.second); } template vector sort_unique(vector vec) { sort(vec.begin(), vec.end()), vec.erase(unique(vec.begin(), vec.end()), vec.end()); return vec; } template int arglb(const std::vector &v, const T &x) { return std::distance(v.begin(), std::lower_bound(v.begin(), v.end(), x)); } template int argub(const std::vector &v, const T &x) { return std::distance(v.begin(), std::upper_bound(v.begin(), v.end(), x)); } template istream &operator>>(istream &is, vector &vec) { for (auto &v : vec) is >> v; return is; } template ostream &operator<<(ostream &os, const vector &vec) { os << '['; for (auto v : vec) os << v << ','; os << ']'; return os; } template ostream &operator<<(ostream &os, const array &arr) { os << '['; for (auto v : arr) os << v << ','; os << ']'; return os; } #if __cplusplus >= 201703L template istream &operator>>(istream &is, tuple &tpl) { std::apply([&is](auto &&... args) { ((is >> args), ...);}, tpl); return is; } template ostream &operator<<(ostream &os, const tuple &tpl) { os << '('; std::apply([&os](auto &&... args) { ((os << args << ','), ...);}, tpl); return os << ')'; } #endif template ostream &operator<<(ostream &os, const deque &vec) { os << "deq["; for (auto v : vec) os << v << ','; os << ']'; return os; } template ostream &operator<<(ostream &os, const set &vec) { os << '{'; for (auto v : vec) os << v << ','; os << '}'; return os; } template ostream &operator<<(ostream &os, const unordered_set &vec) { os << '{'; for (auto v : vec) os << v << ','; os << '}'; return os; } template ostream &operator<<(ostream &os, const multiset &vec) { os << '{'; for (auto v : vec) os << v << ','; os << '}'; return os; } template ostream &operator<<(ostream &os, const unordered_multiset &vec) { os << '{'; for (auto v : vec) os << v << ','; os << '}'; return os; } template ostream &operator<<(ostream &os, const pair &pa) { os << '(' << pa.first << ',' << pa.second << ')'; return os; } template ostream &operator<<(ostream &os, const map &mp) { os << '{'; for (auto v : mp) os << v.first << "=>" << v.second << ','; os << '}'; return os; } template ostream &operator<<(ostream &os, const unordered_map &mp) { os << '{'; for (auto v : mp) os << v.first << "=>" << v.second << ','; os << '}'; return os; } #ifdef HITONANODE_LOCAL const string COLOR_RESET = "\033[0m", BRIGHT_GREEN = "\033[1;32m", BRIGHT_RED = "\033[1;31m", BRIGHT_CYAN = "\033[1;36m", NORMAL_CROSSED = "\033[0;9;37m", RED_BACKGROUND = "\033[1;41m", NORMAL_FAINT = "\033[0;2m"; #define dbg(x) cerr << BRIGHT_CYAN << #x << COLOR_RESET << " = " << (x) << NORMAL_FAINT << " (L" << __LINE__ << ") " << __FILE__ << COLOR_RESET << endl #define dbgif(cond, x) ((cond) ? cerr << BRIGHT_CYAN << #x << COLOR_RESET << " = " << (x) << NORMAL_FAINT << " (L" << __LINE__ << ") " << __FILE__ << COLOR_RESET << endl : cerr) #else #define dbg(x) (x) #define dbgif(cond, x) 0 #endif // Range Minimum Query for static sequence by sparse table // Complexity: (N \log N)$ for precalculation, (1)$ per query template struct StaticRMQ { inline T func(const T &l, const T &r) const noexcept { return std::min(l, r); } int N, lgN; T defaultT; std::vector> data; std::vector lgx_table; StaticRMQ() = default; StaticRMQ(const std::vector &sequence, T defaultT) : N(sequence.size()), defaultT(defaultT) { lgx_table.resize(N + 1); for (int i = 2; i < N + 1; i++) lgx_table[i] = lgx_table[i >> 1] + 1; lgN = lgx_table[N] + 1; data.assign(lgN, std::vector(N, defaultT)); data[0] = sequence; for (int d = 1; d < lgN; d++) { for (int i = 0; i + (1 << d) <= N; i++) { data[d][i] = func(data[d - 1][i], data[d - 1][i + (1 << (d - 1))]); } } } T get(int l, int r) const { // [l, r), 0-indexed assert(l >= 0 and r <= N); if (l >= r) return defaultT; int d = lgx_table[r - l]; return func(data[d][l], data[d][r - (1 << d)]); } }; struct TreeLCA { const int N; std::vector> to; bool built; TreeLCA(int V = 0) : N(V), to(V), built(false) {} void add_edge(int u, int v) { assert(0 <= u and u < N); assert(0 <= v and v < N); assert(u != v); to[u].push_back(v); to[v].push_back(u); } using P = std::pair; std::vector subtree_begin; std::vector

vis_order; std::vector depth; void _build_dfs(int now, int prv, int dnow) { subtree_begin[now] = vis_order.size(); vis_order.emplace_back(dnow, now); depth[now] = dnow; for (auto &&nxt : to[now]) { if (nxt != prv) { _build_dfs(nxt, now, dnow + 1); vis_order.emplace_back(dnow, now); } } } StaticRMQ

rmq; void build(int root = 0) { assert(root >= 0 and root < N); built = true; subtree_begin.resize(N); vis_order.reserve(N * 2); depth.resize(N); _build_dfs(root, -1, 0); rmq = {vis_order, P{N, -1}}; } int lca(int u, int v) { assert(0 <= u and u < N); assert(0 <= v and v < N); if (!built) build(); auto a = subtree_begin[u], b = subtree_begin[v]; if (a > b) std::swap(a, b); return rmq.get(a, b + 1).second; }; int path_length(int u, int v) { return depth[u] + depth[v] - depth[lca(u, v)] * 2; } }; // UnionFind Tree (0-indexed), based on size of each disjoint set struct UnionFind { std::vector par, cou; UnionFind(int N = 0) : par(N), cou(N, 1) { iota(par.begin(), par.end(), 0); } int find(int x) { return (par[x] == x) ? x : (par[x] = find(par[x])); } bool unite(int x, int y) { x = find(x), y = find(y); if (x == y) return false; if (cou[x] < cou[y]) std::swap(x, y); par[y] = x, cou[x] += cou[y]; return true; } int count(int x) { return cou[find(x)]; } bool same(int x, int y) { return find(x) == find(y); } }; int main() { int N; cin >> N; vector> edges; TreeLCA graph(N); int ng = -1, ok = 0; vector> to(N); REP(i, N - 1) { int a, b, c; cin >> a >> b >> c; --a, --b; edges.emplace_back(a, b, c); chmax(ok, c); graph.add_edge(a, b); to[a].emplace_back(b, c); to[b].emplace_back(a, c); } const int maxi = ok; const int root = 0; graph.build(root); vector par(N, pint(-1, -1)); auto rec = [&](auto &&self, int now, int prv) -> void { for (auto [nxt, w] : to[now]) { if (nxt == prv) continue; par[nxt] = pint(now, w); self(self, nxt, now); } }; rec(rec, root, -1); dbg(par); while (ok > ng + 1) { const int m = (ok + ng) / 2; UnionFind uf(N); vector dvs; int lca = -1; for (auto [a, b, w] : edges) { if (w > m) { if (lca < 0) lca = a; lca = graph.lca(lca, a); lca = graph.lca(lca, b); uf.unite(a, b); dvs.emplace_back(graph.depth[a], a); dvs.emplace_back(graph.depth[b], b); } } dvs = sort_unique(dvs); reverse(ALL(dvs)); vector vis(N); vector deg(N); if (lca >= 0) deg[lca] -= 1; bool failed = false; for (auto [d, v] : dvs) { if (vis[v]) continue; vis[v] = 1; while (v != lca) { if (par[v].second < maxi - m) { failed = true; break; } v = par[v].first; deg[v]++; vis[v] = 1; } } dbgif(m == 8, vis); dbgif(m == 8, deg); dbgif(m == 8, failed); dbgif(m == 8, dvs); if (*max_element(ALL(deg)) > 1) failed = true; (failed ? ng : ok) = m; } cout << ok << '\n'; }