AtCoder Regular Contest 109 参加記

C - Large RPS Tournament

C - Large RPS Tournament

[l, r) の結果を全ての l, r について求めたいが、これは実は状態数がかなり少ないので、愚直なメモ化再帰を書くだけで通る。

use std::collections::BTreeMap;

fn main() {
    let (r, w) = (std::io::stdin(), std::io::stdout());
    let mut sc = IO::new(r.lock(), w.lock());

    let n: usize = sc.read();
    let k: usize = sc.read();
    let s = sc.chars();
    let (win, _) = dp(0, k, &s, &mut BTreeMap::new());
    println!("{}", win);
}

fn dp(
    offset: usize,
    k: usize,
    s: &[char],
    memo: &mut BTreeMap<(usize, usize), (char, usize)>,
) -> (char, usize) {
    if let Some(&(win, offset)) = memo.get(&(offset, k)) {
        return (win, offset);
    }
    if k == 1 {
        let left = s[offset];
        let right = s[(offset + 1) % s.len()];
        let next_offset = (offset + 2) % s.len();
        let win = pick_win(left, right);
        return (win, next_offset);
    }

    let (left, next_offset) = dp(offset, k - 1, s, memo);
    let (right, next_offset) = dp(next_offset, k - 1, s, memo);
    let win = pick_win(left, right);
    memo.insert((offset, k), (win, next_offset));
    (win, next_offset)
}

fn pick_win(left: char, right: char) -> char {
    if left == right {
        left
    } else {
        match (left, right) {
            ('R', 'S') | ('S', 'R') => 'R',
            ('S', 'P') | ('P', 'S') => 'S',
            ('P', 'R') | ('R', 'P') => 'P',
            _ => unreachable!(),
        }
    }
}

pub struct IO<R, W: std::io::Write>(R, std::io::BufWriter<W>);

impl<R: std::io::Read, W: std::io::Write> IO<R, W> {
    pub fn new(r: R, w: W) -> Self {
        Self(r, std::io::BufWriter::new(w))
    }
    pub fn write<S: ToString>(&mut self, s: S) {
        use std::io::Write;
        self.1.write_all(s.to_string().as_bytes()).unwrap();
    }
    pub fn read<T: std::str::FromStr>(&mut self) -> T {
        use std::io::Read;
        let buf = self
            .0
            .by_ref()
            .bytes()
            .map(|b| b.unwrap())
            .skip_while(|&b| b == b' ' || b == b'\n' || b == b'\r' || b == b'\t')
            .take_while(|&b| b != b' ' && b != b'\n' && b != b'\r' && b != b'\t')
            .collect::<Vec<_>>();
        unsafe { std::str::from_utf8_unchecked(&buf) }
            .parse()
            .ok()
            .expect("Parse error.")
    }
    pub fn vec<T: std::str::FromStr>(&mut self, n: usize) -> Vec<T> {
        (0..n).map(|_| self.read()).collect()
    }
    pub fn chars(&mut self) -> Vec<char> {
        self.read::<String>().chars().collect()
    }
}

結果

• 赤パフォ: ABCE 72:29
• 橙パフォ: ABCD 96:51
• 黄パフォ: ABC 36:15
• 自分: ABC 62:03

感想

結果的には（自分の実力帯では）ABCの３問早解き勝負だったのだが、そこで勝てなかった。
早解きをするか、あと1問を頑張るしかないが…