#1903. 袭击

https://sjzezoj.com/submission/68864

S2OJ/1903/1903.cpp

@@ -0,0 +1,66 @@
+#include <iostream>
+#include <numeric>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 5005;
+const int mod = 998244353;
+
+int n, m, l[N], r[N], f[N][N];
+int fac[N], inv[N], fac_inv[N];
+
+int A(int n, int m) {
+    if (n < m || n < 0 || m < 0) return 0;
+    return static_cast<long long>(fac[n]) * fac_inv[n - m] % mod;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    fac[0] = 1;
+    for (int i = 1; i < N; i++) {
+        fac[i] = static_cast<long long>(fac[i - 1]) * i % mod;
+    }
+
+    inv[0] = inv[1] = 1;
+    for (int i = 2; i < N; i++) {
+        inv[i] = static_cast<long long>(mod - (mod / i)) * inv[mod % i] % mod;
+    }
+
+    fac_inv[0] = fac_inv[1] = 1;
+    for (int i = 2; i < N; i++) {
+        fac_inv[i] = static_cast<long long>(fac_inv[i - 1]) * inv[i] % mod;
+    }
+
+    cin >> n >> m;
+
+    for (int i = 1, x, y; i <= n; i++) {
+        cin >> x >> y;
+
+        l[x]++, r[y]++;
+    }
+
+    std::partial_sum(l + 1, l + 1 + m, l + 1);
+    std::partial_sum(r + 1, r + 1 + m, r + 1);
+
+    f[0][0] = 1;
+
+    for (int i = 1; i <= m; i++) {
+        for (int j = 0; j <= n; j++) {
+            f[i][j] = f[i - 1][j];
+
+            if (j - 1 >= 0 && r[i] - j + 1 >= 1) {
+                f[i][j] = (static_cast<long long>(f[i][j]) + static_cast<long long>(f[i - 1][j - 1]) * (r[i] - j + 1) % mod + mod) % mod;
+            }
+
+            f[i][j] = static_cast<long long>(f[i][j]) * A(i - j - l[i - 1], l[i] - l[i - 1]) % mod;
+        }
+    }
+
+    cout << f[m][n] << endl;
+
+    return 0;
+}