P4149 [IOI2011]Race

https://www.luogu.com.cn/record/76341077

Luogu/P4149/P4149.cpp

@@ -0,0 +1,116 @@
+#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;
+}