中缀表达式计算器

一、实现思路

1.双栈，一个操作符栈，一个操作数栈，从左到右扫描所有的字符
2.如果是操作数，直接入操作数栈；如果是操作符，比较栈顶操作符和新操作符优先级
如果新的操作符是左括号或优先级高于栈顶元素时新的操作符入栈；
如果新的操作符优先级不高于栈顶元素的，就先出栈一个操作符（如果新操作符是右括号要循环弹出操符直到碰到左括号结束和两个操作数计算并将结果入操作数栈，新操作符再入栈；
3.直到所有的字符都扫描完毕，然后不断弹出操作符栈计算，直到栈为空.弹出并返回操作符栈中仅有的元素

二、具体实现

1.定义一个操作符优先级枚举

/**
 * @ClassName: OperatorEnum
 * @description: 操作符枚举
 * @author: leijing
 * @Date: 2019/4/13 下午10:13
 * @Version: 1.0
 */
public enum  OperatorEnum {
    RB(')',0),LB('(',1),ADD('+',2),SUB('-',2),MUL('*',3),DIV('/',3);
    private int priority;
    private char name;
    OperatorEnum(char name , int priority){
        this.name = name;
        this.priority = priority;
    }

    public static OperatorEnum getOp(char ch){
        OperatorEnum[] values = OperatorEnum.values();
        for (OperatorEnum op : values){
            if (op.name == ch){
                return  op;
            }
        }
        return  null;
    }

    public int getPriority() {
        return priority;
    }

    public char getName() {
        return name;
    }
}

2.定义一个表达式计算器


import java.util.Stack;

/**
 * @ClassName: ExpressCalculator
 * @description: 中缀表达式计算器
 * @author: leijing
 * @Date: 2019/4/13 下午10:12
 * @Version: 1.0
 */
public class ExpressCalculator {
    public int calculateExpress(String express) {
        Stack<Integer> numStack = new Stack<>();
        Stack<OperatorEnum> opStack = new Stack<>();
        for (int i = 0; i < express.length(); i++) {
            char ch = express.charAt(i);
            OperatorEnum op = OperatorEnum.getOp(ch);
            if (op != null) {//是操作符
                if (opStack.empty() || op.equals(OperatorEnum.LB)
                        || opStack.peek().getPriority() < op.getPriority()) {//如果操作符栈为空或者新的操作符优先级大于栈顶元素的 入操作符栈
                    opStack.push(op);
                } else {
                    if (OperatorEnum.RB.equals(op)) {//新操作符如果是')' 需要再弹出一个操作符
                        while (!opStack.empty() && !opStack.peek().equals(OperatorEnum.LB)) {
                            int result = calculate(opStack.pop().getName(), numStack.pop(), numStack.pop());
                            numStack.push(result);//新的结果入操作符栈
                        }
                        opStack.pop();//弹出左括号
                    } else {
                        int result = calculate(opStack.pop().getName(), numStack.pop(), numStack.pop());
                        numStack.push(result);//新的结果入操作符栈
                        opStack.push(op);//新的操作符入栈
                    }

                }
            } else {//是数字
                int num = ch - '0';
                numStack.push(num);
            }
        }
        while (!opStack.empty()) {
            OperatorEnum top = opStack.pop();
            int result = calculate(top.getName(), numStack.pop(), numStack.pop());
            numStack.push(result);
        }
        return numStack.pop();
    }

    private int calculate(char ch, int num2, int num1) {
        int result = 0;
        switch (ch) {
            case '+': {
                result = num1 + num2;
                break;
            }
            case '-': {
                result = num1 - num2;
                break;
            }
            case '*': {
                result = num1 * num2;
                break;
            }
            case '/': {
                result = num1 / num2;
                break;
            }
        }
        return result;
    }
}

3.写一个单元测试类



import org.junit.Assert;
import org.junit.Test;
import org.express.ExpressCalculator;

/**
 * Unit test for ExpressCalculator.
 */
public class ExpressCalculatorTest
{

    @Test
    public void calculateExpress() {
        ExpressCalculator calculator = new ExpressCalculator();
        String express = "1+2+3*4*5";
        int result = calculator.calculateExpress(express);
        System.out.println(express+"="+result);
        Assert.assertEquals(63 , result);
    }

    @Test
    public void calculateExpress2() {
        ExpressCalculator calculator = new ExpressCalculator();
        String express = "(8-1)*(5+5)/7";
        int result = calculator.calculateExpress(express);
        System.out.println(express + "=" + result);
        Assert.assertEquals(10 , result);
    }
    @Test
    public void calculateExpress4() {
        ExpressCalculator calculator = new ExpressCalculator();
        String express = "5+(4*(3+2))";
        int result = calculator.calculateExpress(express);
        System.out.println(express + "=" + result);
        Assert.assertEquals(25 , result);
    }
    }