提出 #52134565

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ソースコード 拡げる

// LUOGU_RID: 154733952
#include <bits/stdc++.h>
using namespace std; 
const int P = 998244353; 
const int N = 2e7 + 5; 

inline int poww(int a, int b) {
    int r = 1; 
    for (; b; b >>= 1, a = 1ll * a * a % P) if (b & 1) r = 1ll * r * a % P; 
    return r; 
}
int n, A, B; 
int fac[N], ifac[N]; 
inline void initFac(void) {
    for (int i = fac[0] = 1; i < N; ++i) fac[i] = 1ll * fac[i - 1] * i % P; 
    ifac[N - 1] = poww(fac[N - 1], P - 2); 
    for (int i = N - 2; i >= 0; --i) ifac[i] = 1ll * ifac[i + 1] * (i + 1) % P; 
}
inline int C(int n, int m) {
    if (m < 0 || n < m || n < 0) return 0; 
    return 1ll * fac[n] * ifac[m] % P * ifac[n - m] % P; 
}
inline int F(int a, int b) {
    return (C(a + b - 1, a - 1) - C(a + b - 1, b - 1) + P) % P; 
}

int main(void) {
    initFac(); 
    cin >> n >> B >> A; // A >= B
    if (A == 0 && B == 0) return cout << "1\n", 0; 
    int ans = (A + B == n ? F(A, B) : 0); 
    for (int k = 0; k <= B; ++k)
        // B - k 次 up
        ans = (ans + 1ll * F(A, B - k) * C(n - A - (B - k) - 1 + k, k)) % P; 
    cout << ans << '\n'; 
    return 0;
}

提出情報

ジャッジ結果