'commit'

TangDou/AcWing/DP/Tree/P3349.cpp

@@ -0,0 +1,67 @@
+#include <bits/stdc++.h>
+using namespace std;
+const int N = 20, M = N << 1;
+#define int long long
+
+// 链式前向星
+int e[M], h[N], idx, w[M], ne[M];
+void add(int a, int b) {
+    e[idx] = b, ne[idx] = h[a], h[a] = idx++;
+}
+
+int g[N][N];
+int f[N][N];
+int st[N];
+int flag[N];
+int n, m;
+// https://blog.csdn.net/Emm_Titan/article/details/123597082
+
+void dfs(int u) {
+    st[u] = 1;
+    for (int i = 1; i <= n; i++) f[u][i] = flag[i] ^ 1;
+
+    for (int i = h[u]; ~i; i = ne[i]) {
+        int v = e[i];
+        if (st[v]) continue;
+        dfs(v);
+
+        for (int i = 1; i <= n; i++) {
+            if (flag[i]) continue;
+            int sum = 0;
+            for (int j = 1; j <= n; j++)
+                if (g[i][j] and !flag[j]) sum += f[v][j];
+            f[u][i] *= sum;
+        }
+    }
+}
+
+signed main() {
+    memset(h, -1, sizeof h);
+    cin >> n >> m;
+    int a, b;
+    for (int i = 1; i <= m; i++) {
+        cin >> a >> b;
+        g[a][b] = g[b][a] = 1;
+    }
+    for (int i = 1; i < n; i++) {
+        cin >> a >> b;
+        add(a, b), add(a, b);
+    }
+    int ans = 0;
+    for (int s = 0; s < (1 << n); s++) {
+        for (int i = 0; i < n; i++)
+            flag[i + 1] = s >> i & 1;
+
+        // 多次调用dfs
+        memset(st, 0, sizeof st);
+        dfs(1);
+
+        int sum = 0;
+        for (int i = 1; i <= n; i++) sum += f[1][i];
+        if (__builtin_popcount(s) & 1) // 似乎是查询s里有多少个数字1
+            ans -= sum;
+        else
+            ans += sum;
+    }
+    printf("%lld\n", ans);
+}