AtCoder Regular Contest 077 E - guruguru

問題

https://beta.atcoder.jp/contests/arc077/tasks/arc077_c

解法

x を設定することによって切り替えが高速化している区間を持っておく。 x を 1 増加させるとそれらの区間の切り替え時間はそれぞれ 1 ずつ短くなる。

コード

use std::cmp;
use std::collections::BinaryHeap;

#[derive(Copy, Clone, Eq, PartialEq, Debug)]
struct Seg {
    head: usize,
    tail: usize,
}

impl Ord for Seg {
    fn cmp(&self, other: &Seg) -> cmp::Ordering {
        other.tail.cmp(&self.tail)
    }
}

impl PartialOrd for Seg {
    fn partial_cmp(&self, other: &Seg) -> Option<cmp::Ordering> {
        Some(self.cmp(other))
    }
}

fn main() {
    let mut sc = Scanner::new();
    let n = sc.read();
    let m = sc.read();
    let a: Vec<usize> = sc.read_vec(n);

    let mut tails: Vec<Vec<usize>> = vec![vec![]; m];
    let mut total_distance = 0;
    let mut current_decrease = 0;
    let mut decreased_segments = BinaryHeap::new();
    for i in 1..n {
        let from = a[i - 1] - 1;
        let to = a[i] - 1;
        let distance = (to + m - from) % m;
        total_distance += distance;
        if from > to {
            let warped = m - from - 1;
            current_decrease += warped;
            decreased_segments.push(Seg {
                head: from,
                tail: to,
            });
            tails[from].push(to + m);
        } else {
            tails[from].push(to);
        }
    }

    let mut ans = total_distance - current_decrease;
    for x in 1..m {
        while let Some(Seg { head, tail }) = decreased_segments.pop() {
            if tail < x {
                assert!(tail == x - 1);
                current_decrease -= (tail + m - head) % m - 1;
            } else {
                decreased_segments.push(Seg {
                    head: head,
                    tail: tail,
                });
                break;
            }
        }
        current_decrease += decreased_segments.len();
        ans = cmp::min(ans, total_distance - current_decrease);

        for &tail in tails[x - 1].iter() {
            decreased_segments.push(Seg {
                head: x - 1,
                tail: tail,
            });
        }
    }
    println!("{}", ans);
}

struct Scanner {
    ptr: usize,
    length: usize,
    buf: Vec<u8>,
    small_cache: Vec<u8>,
}

#[allow(dead_code)]
impl Scanner {
    fn new() -> Scanner {
        Scanner {
            ptr: 0,
            length: 0,
            buf: vec![0; 1024],
            small_cache: vec![0; 1024],
        }
    }

    fn load(&mut self) {
        use std::io::Read;
        let mut s = std::io::stdin();
        self.length = s.read(&mut self.buf).unwrap();
    }

    fn byte(&mut self) -> u8 {
        if self.ptr >= self.length {
            self.ptr = 0;
            self.load();
            if self.length == 0 {
                self.buf[0] = b'\n';
                self.length = 1;
            }
        }

        self.ptr += 1;
        return self.buf[self.ptr - 1];
    }

    fn is_space(b: u8) -> bool {
        b == b'\n' || b == b'\r' || b == b'\t' || b == b' '
    }

    fn read_vec<T>(&mut self, n: usize) -> Vec<T>
    where
        T: std::str::FromStr,
        T::Err: std::fmt::Debug,
    {
        (0..n).map(|_| self.read()).collect()
    }

    fn usize_read(&mut self) -> usize {
        self.read()
    }

    fn read<T>(&mut self) -> T
    where
        T: std::str::FromStr,
        T::Err: std::fmt::Debug,
    {
        let mut b = self.byte();
        while Scanner::is_space(b) {
            b = self.byte();
        }

        for pos in 0..self.small_cache.len() {
            self.small_cache[pos] = b;
            b = self.byte();
            if Scanner::is_space(b) {
                return String::from_utf8_lossy(&self.small_cache[0..(pos + 1)])
                    .parse()
                    .unwrap();
            }
        }

        let mut v = self.small_cache.clone();
        while !Scanner::is_space(b) {
            v.push(b);
            b = self.byte();
        }
        return String::from_utf8_lossy(&v).parse().unwrap();
    }
}