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