// ---------- begin ModInt ---------- mod modint { #[allow(dead_code)] pub struct Mod; impl ConstantModulo for Mod { const MOD: u32 = 998_244_353; } #[allow(dead_code)] pub struct StaticMod; static mut STATIC_MOD: u32 = 0; impl Modulo for StaticMod { fn modulo() -> u32 { unsafe { STATIC_MOD } } } #[allow(dead_code)] impl StaticMod { pub fn set_modulo(p: u32) { unsafe { STATIC_MOD = p; } } } use std::marker::*; use std::ops::*; pub trait Modulo { fn modulo() -> u32; } pub trait ConstantModulo { const MOD: u32; } impl<T> Modulo for T where T: ConstantModulo, { fn modulo() -> u32 { T::MOD } } pub struct ModInt<T>(pub u32, PhantomData<T>); impl<T> Clone for ModInt<T> { fn clone(&self) -> Self { ModInt::new_unchecked(self.0) } } impl<T> Copy for ModInt<T> {} impl<T: Modulo> Add for ModInt<T> { type Output = ModInt<T>; fn add(self, rhs: Self) -> Self::Output { let mut d = self.0 + rhs.0; if d >= T::modulo() { d -= T::modulo(); } ModInt::new_unchecked(d) } } impl<T: Modulo> AddAssign for ModInt<T> { fn add_assign(&mut self, rhs: Self) { *self = *self + rhs; } } impl<T: Modulo> Sub for ModInt<T> { type Output = ModInt<T>; fn sub(self, rhs: Self) -> Self::Output { let mut d = self.0 - rhs.0; if d >= T::modulo() { d += T::modulo(); } ModInt::new_unchecked(d) } } impl<T: Modulo> SubAssign for ModInt<T> { fn sub_assign(&mut self, rhs: Self) { *self = *self - rhs; } } impl<T: Modulo> Mul for ModInt<T> { type Output = ModInt<T>; fn mul(self, rhs: Self) -> Self::Output { let v = self.0 as u64 * rhs.0 as u64 % T::modulo() as u64; ModInt::new_unchecked(v as u32) } } impl<T: Modulo> MulAssign for ModInt<T> { fn mul_assign(&mut self, rhs: Self) { *self = *self * rhs; } } impl<T: Modulo> Neg for ModInt<T> { type Output = ModInt<T>; fn neg(self) -> Self::Output { if self.0 == 0 { Self::zero() } else { Self::new_unchecked(T::modulo() - self.0) } } } impl<T> std::fmt::Display for ModInt<T> { fn fmt<'a>(&self, f: &mut std::fmt::Formatter<'a>) -> std::fmt::Result { write!(f, "{}", self.0) } } impl<T> std::fmt::Debug for ModInt<T> { fn fmt<'a>(&self, f: &mut std::fmt::Formatter<'a>) -> std::fmt::Result { write!(f, "{}", self.0) } } impl<T: Modulo> std::str::FromStr for ModInt<T> { type Err = std::num::ParseIntError; fn from_str(s: &str) -> Result<Self, Self::Err> { let val = s.parse::<u32>()?; Ok(ModInt::new(val)) } } impl<T: Modulo> From<usize> for ModInt<T> { fn from(val: usize) -> ModInt<T> { ModInt::new_unchecked((val % T::modulo() as usize) as u32) } } impl<T: Modulo> From<u64> for ModInt<T> { fn from(val: u64) -> ModInt<T> { ModInt::new_unchecked((val % T::modulo() as u64) as u32) } } impl<T: Modulo> From<i64> for ModInt<T> { fn from(val: i64) -> ModInt<T> { let m = T::modulo() as i64; ModInt::new((val % m + m) as u32) } } #[allow(dead_code)] impl<T> ModInt<T> { pub fn new_unchecked(d: u32) -> Self { ModInt(d, PhantomData) } pub fn zero() -> Self { ModInt::new_unchecked(0) } pub fn one() -> Self { ModInt::new_unchecked(1) } pub fn is_zero(&self) -> bool { self.0 == 0 } } #[allow(dead_code)] impl<T: Modulo> ModInt<T> { pub fn new(d: u32) -> Self { ModInt::new_unchecked(d % T::modulo()) } pub fn pow(&self, mut n: u64) -> Self { let mut t = Self::one(); let mut s = *self; while n > 0 { if n & 1 == 1 { t *= s; } s *= s; n >>= 1; } t } pub fn inv(&self) -> Self { assert!(self.0 != 0); self.pow(T::modulo() as u64 - 2) } } } // ---------- end ModInt ---------- // ---------- begin Precalc ---------- mod precalc { use super::modint::*; #[allow(dead_code)] pub struct Precalc<T> { inv: Vec<ModInt<T>>, fact: Vec<ModInt<T>>, ifact: Vec<ModInt<T>>, } #[allow(dead_code)] impl<T: Modulo> Precalc<T> { pub fn new(n: usize) -> Precalc<T> { let mut inv = vec![ModInt::one(); n + 1]; let mut fact = vec![ModInt::one(); n + 1]; let mut ifact = vec![ModInt::one(); n + 1]; for i in 2..(n + 1) { fact[i] = fact[i - 1] * ModInt::new_unchecked(i as u32); } ifact[n] = fact[n].inv(); if n > 0 { inv[n] = ifact[n] * fact[n - 1]; } for i in (1..n).rev() { ifact[i] = ifact[i + 1] * ModInt::new_unchecked((i + 1) as u32); inv[i] = ifact[i] * fact[i - 1]; } Precalc { inv: inv, fact: fact, ifact: ifact, } } pub fn inv(&self, n: usize) -> ModInt<T> { assert!(n > 0); self.inv[n] } pub fn fact(&self, n: usize) -> ModInt<T> { self.fact[n] } pub fn ifact(&self, n: usize) -> ModInt<T> { self.ifact[n] } pub fn perm(&self, n: usize, k: usize) -> ModInt<T> { if k > n { return ModInt::zero(); } self.fact[n] * self.ifact[n - k] } pub fn comb(&self, n: usize, k: usize) -> ModInt<T> { if k > n { return ModInt::zero(); } self.fact[n] * self.ifact[k] * self.ifact[n - k] } } } // ---------- end Precalc ---------- use modint::*; type M = ModInt<Mod>; // ---------- begin input macro ---------- // reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8 macro_rules! input { (source = $s:expr, $($r:tt)*) => { let mut iter = $s.split_whitespace(); input_inner!{iter, $($r)*} }; ($($r:tt)*) => { let s = { use std::io::Read; let mut s = String::new(); std::io::stdin().read_to_string(&mut s).unwrap(); s }; let mut iter = s.split_whitespace(); input_inner!{iter, $($r)*} }; } macro_rules! input_inner { ($iter:expr) => {}; ($iter:expr, ) => {}; ($iter:expr, $var:ident : $t:tt $($r:tt)*) => { let $var = read_value!($iter, $t); input_inner!{$iter $($r)*} }; } macro_rules! read_value { ($iter:expr, ( $($t:tt),* )) => { ( $(read_value!($iter, $t)),* ) }; ($iter:expr, [ $t:tt ; $len:expr ]) => { (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>() }; ($iter:expr, chars) => { read_value!($iter, String).chars().collect::<Vec<char>>() }; ($iter:expr, bytes) => { read_value!($iter, String).bytes().collect::<Vec<u8>>() }; ($iter:expr, usize1) => { read_value!($iter, usize) - 1 }; ($iter:expr, $t:ty) => { $iter.next().unwrap().parse::<$t>().expect("Parse error") }; } // ---------- end input macro ---------- use std::collections::*; type Map<K, V> = HashMap<K, V>; fn run() { input! { n: usize, s: bytes, } let p = 28; let mut dp = vec![vec![M::zero(); p]; p]; dp[26][27] = M::one(); for &s in s.iter().take(2) { let mut next = vec![vec![M::zero(); p]; p]; for (a, dp) in dp.iter().enumerate() { for (b, w) in dp.iter().enumerate() { for c in b'a'..=b'z' { let x = (c - b'a') as usize; if (s == c || s == b'?') && a != x && b != x { next[x][a] += *w; } } } } dp = next; } let p = 26; for dp in dp.iter_mut() { dp.truncate(p); } dp.truncate(p); for &a in s.iter().skip(2) { let c = (a - b'a') as usize; let mut next = vec![vec![M::zero(); p]; p]; for i in 0..p { let s = (0..p).fold(M::zero(), |s, a| s + dp[i][a]); for j in 0..p { if i != j && (a == b'?' || c == j) { next[j][i] = s - dp[i][j]; } } } dp = next; } let ans = dp.iter().flatten().fold(M::zero(), |s, a| s + *a); println!("{}", ans); } fn main() { run(); }