Encryption and decryption analysis, correction on unknow X sum analysis

Encrytion and decryption analysis

Constraints: 
string s with length length, s[0] = 'c', s[1] = 'r'
  
Encryption(s[0]) = (char)s[0] + 1   - refer to line 56 
  
Encryption(s[1]) = s[1] + Encryption(s[0]) - x1 * 26  
  
Encryption(s[2]) = s[2] + Encryption(s[1]) - x2 * 26   
  
Encryption(s[i]) = s[i] + Encryption(s[i - 1]) - xi * 26 -> a - z, a ascii value 97 
                 = 1 + s[0] + s[1] +...+ s[i] - (x0 + x1 + x2 +...+ xi) * 26   --- formula, need to find s[i]
  
  we denote sumSource = s[0] + s[1] + ...+ s[i] -> SumOfSource
            sumUnknownX = x0 + x1 + x2 + ... + xi -> it will O(n) 
  s[i] = Encryption(s[i]) - 1 - SumOfSource(0->i - 1) + sumUnknownX * 26
  
  Time complexity: for each char, one addition, one subtraction, sumUnknownX * 26 -> 
  while loop each time add 26 -> sumUnknownX -> n * 3, O(n^2) -> O(n)    
    -1000 -> 97 - a, number +  (97 - number)/ 26 * 26