Problem - 1312D - Codeforces

题意：

 

思路：

 

 

 

Code：

#include <bits/stdc++.h>

#define int long long

using i64 = long long;

constexpr int N = 2e5 + 10;
constexpr int M = 2e5 + 10;
constexpr int mod = 998244353;
constexpr int Inf = 1e18;

int n, m;
int Fac[N];

int qpow(int a, int b) {
	int res = 1;
	while(b) {
		if (b & 1) res = (res * a) % mod;
		a = (a * a) % mod;
		b >>= 1;
	}
	return res;
}
int inv(int x) {
	return qpow(x, mod - 2) % mod;
}
int C(int n, int m) {
	return Fac[n] * inv(Fac[m]) % mod * inv(Fac[n - m]) % mod;
}
void F_init() {
	Fac[0] = 1;
	for (int i = 1; i < N; i ++) {
		Fac[i] = (Fac[i - 1] * i) % mod;
	}

}
void solve() {
	std::cin >> n >> m;
	std::cout << C(m, n - 1) * (n - 2)  % mod * qpow(2, n - 2) % mod * inv(2) % mod << "\n";
}
signed main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    
    int t = 1;
	F_init();

    while (t--) {
        solve();
    }
    
    return 0;
}