Encryption and decryption - mock interview 10:00 pm - Jan. 24, 2018

EncryptionAndDecryption.cs

using System;

class Solution
{
    public static string Decrypt(string word)  // "dnotq"
    {
      if(word == null || word.Length == 0)  // false
      {
        return string.Empty;
      }
      
      var length = word.Length; // 5
      
      var original = new char[length]; // 
      
      original[0] = (char)(word[0] - 1); // 'c'
      
      long sumOfSource = (int)original[0]; // c
      
      for(int index = 1; index < length; index++)
      {
        var current = word[index]; 
        //s[i] = Encryption(s[i]) - 1 - SumOfSource(0->i - 1) + x * 26
        long diff = word[index] - 1 - sumOfSource;
        
        // add x * 26 to a - z
        //var smaller = diff < 97; 
        /*
        if(smaller)
        {
          // number +  (97 - number)/ 26 * 26
          diff += (97 - diff)/ 26 * 26;  // diff + 26 * x < 97 
                                         // -> 26 * x < (97 - diff)
        }
        */
        while ( diff < 97)
        {
          diff += 26; 
        }
        
        original[index] = (char)diff; 
        
        sumOfSource += original[index];  // in mock interview, a bug is here!
      }
      
      return new string(original); 
    }

    static void Main(string[] args)
    {
      Console.WriteLine(Decrypt("anotq")); 
    }
}

/*
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]) - x * 26  
  
Encryption(s[2]) = s[2] + Encryption(s[1]) - x * 26   
  
Encryption(s[i]) = s[i] + Encryption(s[i - 1]) - x * 26 -> a - z, a ascii value 97 
                 = 1 + s[0] + s[1] +...+ s[i] - x * 26   --- formula, need to find s[i]
  
  we denote sumSource = s[0] + s[1] + ...+ s[i] -> SumOfSource
  
  s[i] = Encryption(s[i]) - 1 - SumOfSource(0->i - 1) + x * 26
  
  Time complexity: for each char, one addition, one subtraction, x * 26 -> while loop each time add 26 -> x -> n * 3, O(n^2) -> O(n)
    
    -1000 -> 97 - a, number +  (97 - number)/ 26 * 26 -> 
  */