leetcode/source/roman_to_integer/mod.rs

pub fn roman_to_int(string: String) -> i32 {
  let mut characters = string.chars().peekable();
  let mut number = 0;

  while let Some(character) = characters.next() {
    let value = match character {
      'I' => 1,
      'V' => 5,
      'X' => 10,
      'L' => 50,
      'C' => 100,
      'D' => 500,
      'M' => 1000,
      _ => unreachable!(),
    };

    if let Some(next) = characters.peek() {
      let subtract = match next {
        'V' | 'X' if character == 'I' => true,
        'L' | 'C' if character == 'X' => true,
        'D' | 'M' if character == 'C' => true,
        _ => false,
      };

      if subtract {
        number -= value;
        continue;
      }
    }

    number += value;
  }

  number
}