递推式

#include <bits/stdc++.h>
using namespace std;
const long long MOD = 998244353;
long long n, k, S[1005][1005];
int main()
{
    cin >> n >> k;
    S[0][0] = 1; // S[~][0]=0
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= i; j++)
            S[i][j] = (S[i - 1][j - 1] + (i - 1) * S[i - 1][j] % MOD) % MOD;
    cout << S[n][k] << "\n";
    return 0;
}