数据结构－栈－计算后缀表达式

package com.jikefriend.socket.datastructure.collection;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 * 计算后缀表达式
 */
public class Postfix
{

    /**
     * 用于存放数字的栈
     */
    final static class StackX
    {
        private int maxSize;
        private int[] stackArray;
        private int top;

        public StackX(int s)
        {
            maxSize = s;
            stackArray = new int[maxSize];
            top = -1;
        }

        public void push(int c)
        {
            stackArray[++top] = c;
        }

        public int pop()
        {
            return stackArray[top--];
        }

        public int peek()
        {
            return stackArray[top];
        }

        public boolean isEmpty()
        {
            return top == -1;
        }

        public int size()
        {
            return top + 1;
        }

        public int peekN(int n)
        {
            return stackArray[n];
        }

        public void displayStack(String s)
        {
            System.out.print(s);
            System.out.print("Stack (bottom --> top): ");
            for (int i = 0; i < size(); i++)
            {
                System.out.print(peekN(i) + " ");
            }
            System.out.println();
        }
    }

    /**
     * 计算后缀表达式的结果
     */
    final static class ParsePost
    {
        private StackX theStack;
        private String input;

        public ParsePost(String s)
        {
            input = s;
        }

        public int doParse()
        {
            theStack = new StackX(20);
            char ch;
            int j;
            int num1, num2, interAns;

            for (j = 0; j < input.length(); j++) {
                ch = input.charAt(j);
                theStack.displayStack("" + ch + "  ");
                if (ch >= '0' && ch <= '9')
                {
                    theStack.push((int)(ch - '0'));
                }
                else
                {
                    num2 = theStack.pop();
                    num1 = theStack.pop();
                    switch (ch)
                    {
                        case '+':
                            interAns = num1 + num2;
                            break;
                        case '-':
                            interAns = num1 - num2;
                            break;
                        case '*':
                            interAns = num1 * num2;
                            break;
                        case '/':
                            interAns = num1 / num2;
                            break;
                        default:
                            interAns = 0;
                            break;
                    }
                    theStack.push(interAns);
                }
            }
            interAns = theStack.pop();
            return interAns;
        }
    }

    public static void main(String[] args) throws IOException{
        String input;
        int output;
        while (true)
        {
            System.out.print("Enter postfix: ");
            System.out.flush();
            input = getString();
            if (input.equals(""))
                break;
            ParsePost theParse = new ParsePost(input);
            output = theParse.doParse();
            System.out.println("Evaluates to " + output);
        }
    }

    public static String getString() throws IOException
    {
        InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        String s = br.readLine();
        return s;
    }

}

摘自《java数据结构与算法(第二版)》 [美] Robert Lafore 著