Submission #53711095

---

Source Code Expand

// LUOGU_RID: 159723041
#include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 5; 
const int P = 998244353; 

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 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 (n < m || m < 0) return 0; 
    return 1ll * fac[n] * ifac[m] % P * ifac[n - m] % P; 
}

int n, k; 

int main(void) {
    initFac(); 
    cin >> n >> k; 
    if (k < n) return cout << "0\n", 0; 
    int ans = 0; 
    for (int R = 0; R <= k; ++R) {
        int B = k - R; 
        if (R < B) continue; 
        if (R >= B + n) { ans = (ans + C(k, R)) % P; continue; }
        if (R == B) --B; 
        ans = (ans + C(R + B, R) - C(R + B, 2 * R - n + 1)) % P; 
    }
    cout << (ans + P) % P << "\n"; 
    return 0;
}

Submission Info

Judge Result