#4455. [ZJOI2016] 小星星

https://hydro.ac/d/bzoj/record/63e0fc84dd8145683f798089

BZOJ/4455/4455.cpp

@@ -0,0 +1,85 @@
+#include <iostream>
+#include <cstring>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 20;
+
+int n, m;
+long long f[N][N], ans;
+bool map[N][N], vis[N];
+std::vector<int> g[N];
+
+void dfs(int u, int fa) {
+    for (int i = 1; i <= n; i++) {
+        if (vis[i]) f[u][i] = 1;
+    }
+
+    for (int v : g[u]) {
+        if (v == fa) continue;
+
+        dfs(v, u);
+
+        for (int i = 1; i <= n; i++) {
+            if (!vis[i]) continue;
+
+            long long t = f[u][i];
+
+            for (int j = 1; j <= n; j++) {
+                if (!vis[j] || !map[i][j]) continue;
+
+                f[u][i] += t * f[v][j];
+            }
+        }
+    }
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n >> m;
+
+    for (int i = 1, u, v; i <= m; i++) {
+        cin >> u >> v;
+
+        map[u][v] = map[v][u] = true;
+    }
+
+    for (int i = 1, u, v; i < n; i++) {
+        cin >> u >> v;
+
+        g[u].emplace_back(v);
+        g[v].emplace_back(u);
+    }
+
+    for (int s = 0; s < 1 << n; s++) {
+        memset(f, 0x00, sizeof(f));
+        memset(vis, 0x00, sizeof(vis));
+
+        for (int i = 1; i <= n; i++) {
+            if ((s >> (i - 1)) & 1) vis[i] = true;
+        }
+
+        dfs(1, -1);
+
+        long long sum = 0;
+
+        for (int i = 1; i <= n; i++) {
+            sum += f[1][i];
+        }
+
+        if ((n - __builtin_popcount(s)) & 1) {
+            ans -= sum;
+        } else {
+            ans += sum;
+        }
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}