def expected_black_ball_position(N: int, K: int) -> int: """ There are N-1 white balls and one black ball arranged in a row, with the black ball initially at the leftmost position. Takahashi performs K operations, where each operation consists of: - Choose two integers a and b uniformly at random between 1 and N, inclusive - If a ≠ b, swap the a-th and b-th balls from the left Find the expected position of the black ball after K operations, modulo 998244353. The result is returned as an integer R where R × Q ≡ P (mod 998244353), where P/Q is the expected value expressed as an irreducible fraction. Args: N: Total number of balls (1 ≤ N ≤ 998244352) K: Number of operations (1 ≤ K ≤ 10^5) Returns: The expected position modulo 998244353 >>> expected_black_ball_position(2, 1) 499122178 >>> expected_black_ball_position(3, 2) 554580198 >>> expected_black_ball_position(4, 4) 592707587 """ MOD = 998244353 inv2 = (MOD + 1) // 2 # 499122177 # Expected value: ((N+1) - (N-1)*((N-2)/N)^K) / 2 # Derived from symmetry: q_K = 1/N + ((N-2)/N)^K * (N-1)/N # E_K = (N+1)/2 - (N-1)/2 * ((N-2)/N)^K ... actually # E_K = (N+1)/2 - (N-1)/2 * r^K with r=(N-2)/N # Equivalent to ((N+1) - (N-1)*r^K)/2 n_mod = N % MOD inv_n = pow(n_mod, MOD - 2, MOD) r = (n_mod - 2) % MOD * inv_n % MOD p = pow(r, K, MOD) return ((n_mod + 1 - (n_mod - 1) * p) % MOD) * inv2 % MOD