using System.Collections.Generic;
public string TransgformString(string s)
var stack = new Stack<char>();
for (var i = 0; i < n; i++)
if (stack.Peek() == 'A' && s[i] == 'B')
else if (stack.Peek() == 'B' && s[i] == 'A')
else if (stack.Peek() == 'C' && s[i] == 'D')
else if (stack.Peek() == 'D' && s[i] == 'C')
var result = new StringBuilder();
result.Append(stack.Pop());
return new string(result.ToString().Reverse().ToArray());