pub fn is_valid(string: String) -> bool {
  // If the input string is of an odd length it cannot be valid, since every
  // opening character must also have a closing one.
  if string.len() % 2 != 0 {
    return false;
  }

  fn get_closing_character(character: char) -> char {
    match character {
      '[' => ']',
      '{' => '}',
      '(' => ')',
      _ => unreachable!(),
    }
  }

  let mut stack = vec![];

  for character in string.chars() {
    match character {
      // For any opening characters, add their counterpart to the stack and move
      // on to the next character in the string.
      '[' | '{' | '(' => stack.push(get_closing_character(character)),

      // For any other characters, get the most recent character we've seen and
      // if it doesn't equal the expected closing character then the string is
      // invalid.
      _ => {
        if character != stack.pop().unwrap_or_default() {
          return false;
        }
      }
    };
  }

  // After the string has been looped over, if anything is left in the stack the
  // string is invalid.
  stack.is_empty()
}