decrypt_message.py

"""
Decrypt Message

Every word is encrypted as follows:
  - Convert every letter to its ASCII value. 
  - Add 1 to the first letter, and then for every letter from the second one to the last one, add the value of the previous letter. 
  - Subtract 26 from every letter until it is in the range of lowercase letters a-z in ASCII. 
  - Convert the values back to letters.

For instance, to encrypt the word “crime”

Decrypted message:  c   r   i   m   e
Step 1:             99  114 105 109 101
Step 2:            100  214 319 428 529
Step 3:            100  110 111 116 113
Encrypted message:  d   n   o   t   q

Write a function named decrypt(word) that receives a string that consists of small latin letters only, and returns the decrypted word.

input:  word = "dnotq"
output: "crime"

input:  word = "flgxswdliefy"
output: "encyclopedia"

"""

# O(n) time
# O(n) space
def decrypt(word):
  decryption = ""
  prev_letter_val = 1
  
  for letter in word:
    letter_ascii_val = ord(letter)
    letter_ascii_val -= prev_letter_val
    
    while letter_ascii_val < ord('a'):
      letter_ascii_val += 26
      
    decryption += chr(letter_ascii_val)
    prev_letter_val += letter_ascii_val

  return decryption