////   This file contains the C++ code from Program 9.21 of//   "Data Structures and Algorithms//    with Object-Oriented Design Patterns in C++"//   by Bruno R. Preiss.////   Copyright (c) 1998 by Bruno R. Preiss, P.Eng.  All rights reserved.////   http://www.pads.uwaterloo.ca/Bruno.Preiss/books/opus4/programs/pgm09_21.cpp//class ExpressionTree : public BinaryTree{public:    ExpressionTree (char c) :	BinaryTree (*new Char (c)) {}};void PostfixToInfix (){    char c;    Stack& stack = *new StackAsLinkedList ();    while (cin >> c, !cin.eof ())    {	if (std::isdigit (c) || std::isalpha (c))	    stack.Push (*new ExpressionTree (c));	else if (c == '+' || c == '-' || c == '*' || c == '/')	{	    ExpressionTree& result = *new ExpressionTree (c);	    result.AttachRight (		dynamic_cast<ExpressionTree&> (stack.Pop ()));	    result.AttachLeft (		dynamic_cast<ExpressionTree&> (stack.Pop ()));	    stack.Push (result);	}    }    ExpressionTree& result =	dynamic_cast<ExpressionTree&> (stack.Pop ());    InfixVisitor visitor;    result.DepthFirstTraversal (visitor);    delete &result;    delete &stack;}