#1541. 【2022.09.07 NOIP 模拟】单调栈

https://sjzezoj.com/submission/55988

S2OJ/1541/1541.cpp

@@ -0,0 +1,71 @@
+#include <iostream>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 305,
+          M = 1e5 + 5;
+const int mod = 1e9 + 7;
+
+int n, m, b[N], fa[N];
+long long f[N][M];
+std::vector<int> g[N];
+bool vis[N];
+
+void dfs(int u) {
+    std::fill_n(f[u] + 1, m + 1, 1);
+
+    for (const int& v : g[u]) {
+        dfs(v);
+
+        for (int j = 1; j <= m + 1; j++) {
+            f[u][j] = (f[u][j] * f[v][j - vis[v]]) % mod;
+        }
+    }
+
+    if (u) {
+        for (int i = 1; i <= m + 1; i++) {
+            f[u][i] = (f[u][i] + f[u][i - 1]) % mod;
+        }
+    }
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n >> m;
+
+    for (int i = 1; i <= n; i++) {
+        cin >> b[i];
+    }
+
+    for (int i = 1; i <= n; i++) {
+        vis[i] = true;
+
+        if (b[i] == -1) {
+            fa[i] = 0;
+        } else {
+            fa[i] = b[i];
+
+            for (int j = b[i] + 1; j < i; j++) {
+                if (fa[j] <= b[i]) {
+                    fa[j] = i;
+                    vis[j] = false;
+                }
+            }
+        }
+    }
+
+    for (int i = 1; i <= n; i++) {
+        g[fa[i]].emplace_back(i);
+    }
+
+    dfs(0);
+
+    cout << f[0][m + 1] << endl;
+
+    return 0;
+}