import java.util.Stack;
import java.util.Scanner;

/* Java class that includes two public methods that pertain to arithmetic
** expressions in prefix form.  One method determines whether or not a 
** given expression is syntactically valid.  The other evaluates a given
** prefix expression.
** 
** Assumed here is that expressions are given in the form of a String
** each token of which is either an unsigned integer literal (e.g., a
** sequence of one or more decimal digits) or one of the operator symbols
** '+', '-', '*', or '/'.  Tokens must be separated by whitespace.
**
** For: CMPS 144, Prog. Assg. #4
** Authors: R. McCloskey and < STUDENT's NAME >
** Collaborated with: ...
** Known Defects: ...
*/
public class PrefixExprEvaluator {

   // class constants
   // ---------------
   public static final String PLUS = "+";
   public static final String MINUS = "-";
   public static final String TIMES = "*";
   public static final String DIVIDED_BY = "/";


   // methods of interest
   // -------------------

   /* Reports whether or not the given string is a syntactically
   ** valid (arithmetic) prefix expression, which is to say that it
   ** satisfies these conditions:
   ** (1) Every token is an unsigned integer literal or an operator.
   ** (2) The number of occurrences of integer literals exceeds the 
   **     number of occurrences of operators by exactly one.
   ** (3) In every proper prefix of the expression, the number of
   **     occurrences of integer literals is less than or equal to 
   **     the number of occurrences of operators.
   **
   ** It is assumed that tokens are separated from each other by
   ** whitespace.
   */
   public boolean isValid(String expr) {
      return false;   // STUB
   }


   /* Evaluates the given arithmetic expression, which is assumed 
   ** to be syntactically valid and in prefix form.
   ** pre: isValid(prefixExpr)
   */
   public int valueOf(String prefixExpr) {
      return -999;   // STUB
   }



   // private utility methods
   // -----------------------

   /* Reports whether or not the given string is equal to one of the
   ** operator symbols.
   */
   private boolean isOperator(String s) {
      return s.equals(PLUS) || s.equals(MINUS) || 
             s.equals(TIMES) || s.equals(DIVIDED_BY);
   }

   /* Returns the given string enclosed in double quotes.
   */
   private String quoted(String s) { 
      final char DOUBLE_QUOTE = '\"';
      return DOUBLE_QUOTE + s + DOUBLE_QUOTE;
   }
   
}