class ExpTreeNode{
public:

  // kinds of nodes which can occur.  the net effect of
  // this declaration is to define a bunch of constants.
  enum nodeType  { PLUS = 0,
                   MINUS = 1,
                   MULTIPLY = 2,
                   DIVIDE = 3,
                   NUMBER = 4,
                   UNDEFINED = 5 };

  nodeType type;
  int value; // only used when “type” == NUMBER
  ExpTreeNode *left, *right;

  // note: the second argument is optional
  ExpTreeNode (nodeType type, int value = 1) {
    ExpTreeNode::type = type;
    ExpTreeNode::value = value; // harmless at worst
    left = right = NULL; // guarantee reasonable values
  }

};


void deleteExpTree(ExpTreeNode *root);

int evaluateExpTree(ExpTreeNode *root);

void outputInfixExpression (ExpTreeNode *root);

void outputPostfixExpression (ExpTreeNode *root);

// outputs an error message and returns null if the String2002 object
// is not a valid expression
ExpTreeNode *createExpTree (String2002 str);