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
}