[백준] 2180번 : 소방서의 고민

https://www.acmicpc.net/problem/2180

다음과 같은 방식으로 정렬조건을 설정하여 정렬하여 해결하면 됩니다.

1. 0번째 값이 0

2. 1번째 값이 0

3. 비교하고자 하는 1번째 값이 모두 0이면, 0번째 값 비교

4. 0번째와 1번째 값 서로 교차하여 곱하고 비교

use io::Write;
use std::{io, str};

pub struct UnsafeScanner<R> {
  reader: R,
  buf_str: Vec<u8>,
  buf_iter: str::SplitAsciiWhitespace<'static>,
}

impl<R: io::BufRead> UnsafeScanner<R> {
  pub fn new(reader: R) -> Self {
    Self {
      reader,
      buf_str: vec![],
      buf_iter: "".split_ascii_whitespace(),
    }
  }

  pub fn token<T: str::FromStr>(&mut self) -> T {
    loop {
      if let Some(token) = self.buf_iter.next() {
        return token.parse().ok().expect("Failed parse");
      }
      self.buf_str.clear();
      self.reader
          .read_until(b'\n', &mut self.buf_str)
          .expect("Failed read");
      self.buf_iter = unsafe {
        let slice = str::from_utf8_unchecked(&self.buf_str);
        std::mem::transmute(slice.split_ascii_whitespace())
      }
    }
  }

  pub fn token_eof<T: str::FromStr>(&mut self) -> Option<T> {
    loop {
      if let Some(token) = self.buf_iter.next() {
        return token.parse().ok();
      }
      self.buf_str.clear();
      if self.reader.read_until(b'\n', &mut self.buf_str).unwrap() == 0 {
        return None;
      }
      self.buf_iter = unsafe {
        let slice = str::from_utf8_unchecked(&self.buf_str);
        std::mem::transmute(slice.split_ascii_whitespace())
      }
    }
  }

  pub fn line(&mut self) -> String {
    let mut input = String::new();
    self.reader.read_line(&mut input).expect("Failed read");
    input
  }
}

fn main() {
  let (stdin, stdout) = (io::stdin(), io::stdout());
  let mut scan = UnsafeScanner::new(stdin.lock());
  let mut out = io::BufWriter::new(stdout.lock());

  let n = scan.token::<usize>();
  let mut arr = (0..n).map(|_| (scan.token::<i64>(), scan.token::<i64>())).collect::<Vec<_>>();

  arr.sort_unstable_by(|a, b| {
    if a.0 == 0 {
      return std::cmp::Ordering::Greater;
    } else if b.0 == 0 {
      return std::cmp::Ordering::Less;
    } else if a.1 == 0 && b.1 == 0 {
      return a.0.cmp(&b.0);
    }
    (a.1 * b.0).cmp(&(b.1 * a.0))
  });

  let mut answer = 0;
  for (x, y) in arr {
    answer = (answer + answer * x + y) % 40000;
  }
  write!(out, "{}", answer).unwrap();
}