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 # Recurrence: with probability p = 2(N-1)/N^2 the black ball moves to a # uniformly random other position, otherwise it stays. # E_{k+1} = (1 - p) E_k + p * (S - E_k)/(N-1), S = N(N+1)/2 # = (1 - 2/N) E_k + (N+1)/N # Fixed point E* = (N+1)/2, so # E_K = (N+1)/2 - (N-1)/2 * (1 - 2/N)^K inv2 = pow(2, MOD - 2, MOD) invN = pow(N % MOD, MOD - 2, MOD) base = (1 - 2 * invN) % MOD ans = (N + 1) % MOD * inv2 % MOD ans = (ans - (N - 1) % MOD * inv2 % MOD * pow(base, K, MOD)) % MOD return ans % MOD