export function findMissingLetter(array: string[]): string {
  const alphabet = 'abcdefghijklmnopqrstuvwxyz';
  // convert array to alphabet index values
  const indexes = array.map((char) => alphabet.indexOf(char.toLowerCase()));
  // find the missing index
  const missingIndex = indexes.find((index, i) => indexes[i + 1] - index > 1) as number;

  const isInputUpperCase = array[0] === array[0].toUpperCase();
  const missingLetter = alphabet[missingIndex + 1];
  return isInputUpperCase ? missingLetter.toUpperCase() : missingLetter;
}