Java.
Must not use Java API java.util.Stack
/** Convert a valid infix expression to a postfixexpression
Must only use variable names as defined in variable table
@param infix : A valid infix expression.
@return Equivalent postfix expression */
/** Evaluates a postfix expression.
Must only use variable names as defined in variable table
@param postfix : A valid postfix expression.
@return The double result of the postfix expression. */
OR
PayPal Gateway not configured
OR
PayPal Gateway not configured