pub fn missing_number(numbers: Vec<i32>) -> i32 {
  // The range of numbers can be described as the total sum of all possible
  // numbers in that range.
  let total_range = (0..=numbers.len() as i32).sum::<i32>();

  // And since only one number is missing from that range, the difference
  // between the range's sum and the numbers' sum will be that missing number.
  let sum_of_numbers = numbers.iter().sum::<i32>();

  total_range - sum_of_numbers
}

#[test]
fn test_missing_number() {
  assert_eq!(missing_number(vec![3, 0, 1]), 2);
  assert_eq!(missing_number(vec![0, 1]), 2);
  assert_eq!(missing_number(vec![9, 6, 4, 2, 3, 5, 7, 0, 1]), 8);
}