letter2num.py

alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

def numToLetter(number):
  result = "";

  charIndex = int(number % len(alphabet))
  quotient = int(number / len(alphabet))

  if (charIndex - 1 == -1):
    charIndex = len(alphabet);
    quotient += 1

  result = alphabet[charIndex - 1] + result;
  if (quotient >= 1):
    result = numToLetter(quotient) + result;

  return result

def letterToNumber(letters):
    result = 0;

    for idx in range(len(letters)):
        char = letters[idx]
        number = alphabet.index(char) + 1
        result = number + result * idx * len(alphabet)

    return result

if __name__ == "__main__":
    for x in [1, 27, 100, 200, 600, 649, 650, 700]:
        print(x, "->", letterToNumber(numToLetter(x)))