public string PostfixToPrefix(string postfixExp)
{
InternalValidateInfixExpresion(postfixExp);
var operands = new Generic.Stack <string>();
var array = postfixExp.ToCharArray();
for (var i = 0; i < array.Length; ++i)
{
var c = array[i];
if (c.IsLetterOrDigit())
{
operands.Push(c.ToString());
continue;
}
if (operands.Count < 2)
{
Throw.InvalidOperationException(Message.OnStack.NumberOfPopOpAreMoreThanElements);
}
var operand_1 = operands.Pop();
operands.Push($"{c}{operands.Pop()}{operand_1}");
}
var prefix = operands.Pop();
operands.Clear();
return(prefix);
}
/// <summary>
/// Converts an infix-expression to postfix-expression.
///
/// Note:- Current character while looping over the input-expression will be described as 'c'.
/// 1. Loop over the input-expression
/// 1. If c is an opening parantheses.
/// 1. Push c to stack.
/// 2. continue;
/// 2. If c is a letter or digit.
/// 1. Append it to expression-builder.
/// 2. continue;
/// 3. If c is an arithmetic-operator
/// 1. Loop over stack untill it is not-empty and c has an equal or lower precedence than the one in the stack
/// 1. If c has equal precedence and is right associative
/// 1. Push the c to stack.
/// 2. continue;
/// 2. If c has equal precedence and is left associative or c has lower precedence than the one in the stack
/// 1. Pop the stack and append its output to expression-builder.
/// 2. Push c on stack.
/// 3. continue;
/// 4. If c is a closing parantheses
/// 1. Loop over stack untill it is not empty and c is not equal to an opening parantheses
/// 1. Pop the stack and append the result to expression-builder.
/// 2. If stack is not empty and stack top is not an opening-parantheses.
/// 1. Throw an exception.
/// 3. Pop the stack.
///
/// 2. Loop over the entire stack untill it is empty
/// 1. Pop the stack and append the result to the expression-builder.
/// </summary>
/// <param name="inputExp">The infix expression.</param>
/// <returns></returns>
public string InfixToPostfix(string inputExp)
{
InternalValidateInfixExpresion(inputExp);
var operators = new Generic.Stack <char>();
var expBuilder = new StringBuilder();
foreach (var c in inputExp)
{
if (c.IsOpeningParantheses())
{
operators.Push(c);
continue;
}
if (c.IsLetterOrDigit())
{
expBuilder.Append(c);
continue;
}
if (c.IsArithmeticOperator())
{
while (operators.IsNotEmpty && (c.HasEqualPrecedenceTo(operators.Peek()) || c.HasLowerPrecedenceThan(operators.Peek())))
{
if (c.HasEqualPrecedenceTo(operators.Peek()) && c.IsRightAssociative())
{
break;
}
if ((c.HasEqualPrecedenceTo(operators.Peek()) && c.IsLeftAssociative()) || c.HasLowerPrecedenceThan(operators.Peek()))
{
expBuilder.Append(operators.Pop());
}
}
operators.Push(c);
continue;
}
if (c.IsClosingParantheses())
{
while (operators.IsNotEmpty && !operators.Peek().IsOpeningParantheses())
{
expBuilder.Append(operators.Pop());
}
if (operators.IsNotEmpty && !operators.Peek().IsOpeningParantheses())
{
Throw.InvalidOperationException(Message.OnStack.InvalidExpression);
}
operators.Pop();
}
}
while (operators.IsNotEmpty)
{
expBuilder.Append(operators.Pop());
}
operators.Clear();
return(expBuilder.ToString(0, expBuilder.Length));
}
/// <summary>
/// Reverses a Stack using extra stack.
/// </summary>
public void ReverseUsingExtraSpace()
{
var stack = new Generic.Stack <T>(Stack.Count);
foreach (var item in Stack)
{
stack.Push(item);
}
Stack = stack;
}