Find all possible strings - each char in the string can be lower case or upper case.

Find all possible strings

/*
abc

output includes 8 strings. 8 combinations of string. 

Abc
ABc
ABC
abc
aBc
abC
aBC
ABC

Please output all possible strings, for each char in the string, there are two choices, 
one is lower case, another one is upper case.
*/

public static void FindAllCases(string s, string prefix, List<string> output) // "abc", "", new List<string>()
{
  // base case
  if(s == null)  // false 
  {
    output.Add(prefix); 
    return; 
  }
  
  var current = s[0]; // 'a'
  var capitalChar = (char)(current - 'a' + 'A'); // 'A'
  
  var nextString = s.substring(1, s.Length - 1); // "bc"
  var nextPrefix = prefix + current.Tostring(); //  "" + "a"
  var nextPrefixCamel = previx + capitalChar.Tostring(); // "" + "A"
  
  FindAllCases(nextString, nextPrefix, output); // "bc", "a", output
  FindAllCases(nextString, nextPrefix, output); // "bc", "A", output
}