/// <summary>
/// Method to convert an Infix expr to Prefix (Polish Notation). This method uses 2 stacks to hold the values.
/// </summary>
/// <param name="input"></param>
/// <returns></returns>
public string ConvertInfixToPrefix(string input)
{
StackUsingLL <int> staging = new StackUsingLL <int>();
StackUsingLL <int> result = new StackUsingLL <int>();
StringBuilder resultString = new StringBuilder();
for (int i = input.Length - 1; i >= 0; i--)
{
if (input[i] == ')')
{
staging.Push(input[i]);
}
else if (IsOperand(input[i]))
{
result.Push(input[i]);
}
else if (input[i] == '(')
{
while (staging.PeekTop() != ')')
{
result.Push(staging.Pop());
}
staging.Pop();
}
else
{
if (!staging.IsEmpty() && CheckPrecedence(input[i]) >= CheckPrecedence(staging.PeekTop()))
{
staging.Push(input[i]);
}
else if (!staging.IsEmpty() && CheckPrecedence(input[i]) < CheckPrecedence(staging.PeekTop()))
{
result.Push(staging.Pop());
staging.Push(input[i]);
}
else if (staging.IsEmpty())
{
staging.Push(input[i]);
}
}
}
while (!staging.IsEmpty())
{
result.Push(staging.Pop());
}
while (!result.IsEmpty())
{
resultString.Append(Convert.ToChar(result.Pop()));
}
return(resultString.ToString());
}
public string ConvertInfixToPostfix(string infix)
{
StackUsingLL <int> staging = new StackUsingLL <int>();
StringBuilder result = new StringBuilder();
for (int i = 0; i < infix.Length; i++)
{
//If scanned character is an operand
if (IsOperand(infix[i]))
{
result.Append(infix[i]);
}
//If scanned character is '(', push it to stack
else if (infix[i] == '(')
{
staging.Push('(');
}
//If scanned character is ')', pop all operators from stack until '(' is found
else if (infix[i] == ')')
{
while (!staging.IsEmpty() && staging.PeekTop() != '(')
{
result.Append(Convert.ToChar(staging.Pop()));
}
//Invalid string check
if (!staging.IsEmpty() && staging.PeekTop() != '(')
{
return(null);
}
else
{
staging.Pop();
}
}
//If operator
else
{
while (!staging.IsEmpty() && (CheckPrecedence(infix[i]) <= CheckPrecedence(Convert.ToChar(staging.PeekTop()))))
{
result.Append(Convert.ToChar(staging.Pop()));
}
staging.Push(infix[i]);
}
}
while (!staging.IsEmpty())
{
result.Append(Convert.ToChar(staging.Pop()));
}
return(result.ToString());
}