java使用栈计算后缀表达式

古城微笑少年丶 2021-11-13 19:10 274阅读 0赞

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"));
        }
    }

转载于:https://www.cnblogs.com/xww115/p/11145127.html