题解 破门而入

传送门

求一行第一类斯特林数
\(O(n^2)\) 能过

Code:

#include <bits/stdc++.h>
using namespace std;
#define INF 0x3f3f3f3f
#define N 3010
#define ll long long
//#define int long long

char buf[1<<21], *p1=buf, *p2=buf;
#define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf, 1, 1<<21, stdin)), p1==p2?EOF:*p1++)
inline int read() {
	int ans=0, f=1; char c=getchar();
	while (!isdigit(c)) {if (c=='-') f=-f; c=getchar();}
	while (isdigit(c)) {ans=(ans<<3)+(ans<<1)+(c^48); c=getchar();}
	return ans*f;
}

int n, k;
const ll mod=998244353;

namespace task1{
	ll s[N][N], ans;
	void solve() {
		s[0][0]=1;
		for (int i=1; i<=n; ++i) {
			for (int j=1; j<=k; ++j) {
				s[i][j]=(s[i-1][j-1]+1ll*(i-1)*s[i-1][j])%mod;
			}
		}
		for (int i=1; i<=k; ++i) ans=(ans+s[n][i])%mod;
		printf("%lld\n", ans);
		exit(0);
	}
}

signed main()
{
	freopen("broken.in", "r", stdin);
	freopen("broken.out", "w", stdout);

	n=read(); k=read();
	task1::solve();

	return 0;
}