P4654 [CEOI2017]Mousetrap

https://www.luogu.com.cn/record/78329033

Luogu/P4654/P4654.cpp

@@ -0,0 +1,91 @@
+#include <iostream>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1e6 + 5;
+
+int n, t, m, fa[N], sum[N], f[N], ans;
+bool vis[N];
+std::vector<int> g[N];
+
+void dfs(int u, int _f) {
+    int max1 = 0, max2 = 0;
+
+    fa[u] = _f;
+
+    if (u != t) {
+        sum[u] = sum[_f] + g[u].size() - 1 - (u != m);
+    }
+
+    for (const int &v : g[u]) {
+        if (v == _f) continue;
+
+        dfs(v, u);
+
+        if (f[v] > max1) {
+            max2 = max1;
+            max1 = f[v];
+        } else if (f[v] > max2) {
+            max2 = f[v];
+        }
+    }
+
+    f[u] = max2 + g[u].size() - 1;
+}
+
+bool check(int x) {
+    for (int i = m, cnt = 1; i != t; i = fa[i], cnt++) {
+        int t = 0;
+
+        for (const int &v : g[i]) {
+            if (vis[v] || sum[i] + f[v] <= x) continue;
+            if (!cnt) return false;
+
+            t++, cnt--;
+        }
+
+        x -= t;
+    }
+
+    return x >= 0;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n >> t >> m;
+
+    for (int i = 1, u, v; i < n; i++) {
+        cin >> u >> v;
+
+        g[u].push_back(v);
+        g[v].push_back(u);
+    }
+
+    dfs(t, 0);
+
+    for (int i = m; i; i = fa[i]) {
+        vis[i] = true;
+    }
+
+    int l = 0, r = n << 1;
+
+    while (l <= r) {
+        int mid = l + r >> 1;
+
+        if (check(mid)) {
+            r = mid - 1;
+            ans = mid;
+        } else {
+            l = mid + 1;
+        }
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}