package com.nps.base.xue.DataStructure.stack.utils;

import java.util.Scanner;

import java.util.Stack;

/*

 * @author XueWeiWei

 * @date 2019/7/7 9:02

 */

public class PostfixEvaluator {

    private final static char ADD = '+';

    private final static char SUBTRACT = '-';

    private final static char MULTIPLY = '*';

    private final static char DIVIDE = '/';

    private Stack<Integer> stack;

    /**

     *  创建一个 evaluator

     */

    public PostfixEvaluator() {

        stack = new Stack<Integer>();

    }

    public int evaluate(String expr){

        int op1,op2, result=0;

        String token;

        Scanner parser = new Scanner(expr);

        while (parser.hasNext()){

            token = parser.next();

            if (isOperator(token)){

                op2 = (stack.pop()).intValue();

                op1 = (stack.pop()).intValue();

                result = evaluateSingleOperator(token.charAt(0),op1,op2);

                stack.push(new Integer(result));

            }else {

                stack.push(new Integer(Integer.parseInt(token)));

            }

        }

        return result;

    }

    private boolean isOperator(String token){

        return (token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/"));

    }

    private int evaluateSingleOperator(char operator, int op1 , int op2){

        int result = 0;

        switch (operator){

            case ADD:

                result = op1 + op2;

                break;

            case SUBTRACT:

                result = op1- op2;

                break;

            case MULTIPLY:

                result = op1 * op2;

                break;

            case DIVIDE:

                result = op1 / op2;

                break;

        }

        return result;

    }

}

package com.nps.base.xue.DataStructure.stack.test;

import com.nps.base.xue.DataStructure.stack.utils.PostfixEvaluator;

import java.util.Scanner;

/*

 * @author XueWeiWei

 * @date 2019/7/7 9:01

 */

public class PostfixTester {

    public static void main(String[] args) {

        String expression,again;

        int result;

        Scanner in = new Scanner(System.in);

        do {

            PostfixEvaluator evaluator = new PostfixEvaluator();

            System.out.println("e.g. 5 4 +  3 2 1 - + *");

            System.out.println();

            expression = in.nextLine();

            System.out.println("输入的表达式：" + expression);

            result = evaluator.evaluate(expression);

            System.out.println(">>>>>结果：  " + result);

            System.out.println("是否输入下一个表达式？[Y/N]");

            again = in.nextLine();

        }while (again.equalsIgnoreCase("y"));

    }

}