java数据结构八、通过栈根据后缀表达式计算值

//postfix.java
//parses postfix arithmetic expressions
//to run this program: C>java PostfixApp
import java.io.*;              // for I/O
////////////////////////////////////////////////////////////////
class StackX
{
private int maxSize;
private int[] stackArray;
private int top;
//--------------------------------------------------------------
public StackX(int size)      // constructor
   {
   maxSize = size;
   stackArray = new int[maxSize];
   top = -1;
   }
//--------------------------------------------------------------
public void push(int j)     // put item on top of stack
   { stackArray[++top] = j; }
//--------------------------------------------------------------
public int pop()            // take item from top of stack
   { return stackArray[top--]; }
//--------------------------------------------------------------
public int peek()           // peek at top of stack
   { return stackArray[top]; }
//--------------------------------------------------------------
public boolean isEmpty()    // true if stack is empty
   { return (top == -1); }
//--------------------------------------------------------------
public boolean isFull()     // true if stack is full
   { return (top == maxSize-1); }
//--------------------------------------------------------------
public int size()           // return size
   { return top+1; }
//--------------------------------------------------------------
public int peekN(int n)     // peek at index n
   { return stackArray[n]; }
//--------------------------------------------------------------
public void displayStack(String s)
   {
   System.out.print(s);
   System.out.print("Stack (bottom-->top): ");
   for(int j=0; j<size(); j++)
      {
      System.out.print( peekN(j) );
      System.out.print(' ');
      }
   System.out.println("");
   }
//--------------------------------------------------------------
}  // end class StackX
////////////////////////////////////////////////////////////////
class ParsePost
{
private StackX theStack;
private String input;
//--------------------------------------------------------------
public ParsePost(String s)
   { input = s; }
//--------------------------------------------------------------
public int doParse()
   {
   theStack = new StackX(20);             // make new stack
   char ch;
   int j;
   int num1, num2, interAns;

   for(j=0; j<input.length(); j++)       // for each char,
      {
      ch = input.charAt(j);              // read from input
      theStack.displayStack(""+ch+" ");  // *diagnostic*
      if(ch >= '0' && ch <= '9')         // if it's a number
         theStack.push( (int)(ch-'0') ); //   push it
      else                               // it's an operator
         {
         num2 = theStack.pop();          // pop operands
         num1 = theStack.pop();
         switch(ch)                      // do arithmetic
            {
            case '+':
               interAns = num1 + num2;
               break;
            case '-':
               interAns = num1 - num2;
               break;
            case '*':
               interAns = num1 * num2;
               break;
            case '/':
               interAns = num1 / num2;
               break;
            default:
               interAns = 0;
            }  // end switch
         theStack.push(interAns);        // push result
         }  // end else
      }  // end for
   interAns = theStack.pop();            // get answer
   return interAns;
   }  // end doParse()
}  // end class ParsePost
////////////////////////////////////////////////////////////////
class PostfixApp
{
public static void main(String[] args) throws IOException
   {
   String input;
   int output;

   while(true)
      {
      System.out.print("Enter postfix: ");
      System.out.flush();
      input = getString();         // read a string from kbd
      if( input.equals("") )       // quit if [Enter]
         break;
                                   // make a parser
      ParsePost aParser = new ParsePost(input);
      output = aParser.doParse();  // do the evaluation
      System.out.println("Evaluates to " + output);
      }  // end while
   }  // end main()
//--------------------------------------------------------------
public static String getString() throws IOException
   {
   InputStreamReader isr = new InputStreamReader(System.in);
   BufferedReader br = new BufferedReader(isr);
   String s = br.readLine();
   return s;
   }
//--------------------------------------------------------------
}  // end class PostfixApp
////////////////////////////////////////////////////////////////