为啥我I题这样写错了，还有这个组合数：fac[n]*inv(fac[m])%mod*inv(fac[n-m])%mod;，这个是啥意思emmm 我的组合数是这么写的： ll C(ll x){ return fac[n] / fac[x] / fac[n - x]; } //以下为源码 #include <iostream> using namespace std; const int N = 1005; typedef long long ll; const int mod = 998244353; ll n, m; ll fac[N]; ll C(ll x){ return fac[n] / fac[x] / fac[n - x]; } ll A(ll x){ return fac[m] / fac[n - x]; } void init(){ fac[0] = 1; for(int i = 1; i < N; i++){ fac[i] = fac[i - 1] * i % mod; } } int main(){ init(); cin >> n >> m; ll ans = 0; ll t = min(n ,m); for(ll i = 0; i <= t; i++){ ans += C(i) * A(i) % mod; ans %= mod; } cout << ans; return 0; }</iostream>