package com.deng.stack;

/**
 * @author 阿卓
 * @version 1.0
 * @date 2020/11/26 0026 8:45
 */
public class Calculator {
    public static void main(String[] args) {
        String expression = "70+2*6-2";
//创建两个栈，数栈，符号栈
        ArrayStack2 numStack = new ArrayStack2(10);
        ArrayStack2 operStack = new ArrayStack2(10);
        //定义需要的相关变量
        int index = 0;//用于扫描
        int num1 = 0;
        int num2 = 0;
        int oper = 0;
        int res = 0;
        char ch = ' ';//将每次扫描得到char保存到ch
        String keepNum = "";//用于拼接多位数
        //开始while循环的扫描expression
        while (true) {
            //依次得到expression 的每一个字符
            ch = expression.substring(index, index + 1).charAt(0);
            //判断ch是什么，然后做相应的处理
            if (operStack.isOper(ch)) {
                //判断当前的符号栈是否为空
                if (!operStack.isEmpty()) {
                    if (operStack.priority(ch) <= operStack.priority(operStack.peek())) {
                        num1 = numStack.pop();
                        num2 = numStack.pop();
                        oper = operStack.pop();
                        res = numStack.cal(num1, num2, oper);
                        //把运算的结果入数栈
                        numStack.push(res);
                        //然后将当前的操作符入符号栈
                        operStack.push(ch);
                    } else {
                        //如果当前的操作符的优先级大于栈中的操作符，就直接入符号栈
                        operStack.push(ch);
                    }
                } else {
                    //如果为空直接入符号栈
                    operStack.push(ch);
                }
            } else {//如果是数，则直接入数栈
//                numStack.push(ch - 48) ;
                keepNum += ch;
                //判断下一个字符是不是数字

                if (index == expression.length() - 1) {
                    numStack.push(Integer.parseInt(keepNum));
                } else {
                    if (operStack.isOper(expression.substring(index + 1, index + 2).charAt(0))) {
                        numStack.push(Integer.parseInt(keepNum));
                        //keepNum清空
                        keepNum = "";
                    }
                }
            }
            // 让index + 1 ,并判断是否扫描到expression最后
            index++;
            if (index >= expression.length()) {
                break;
            }
        }
        //当表达式扫描完毕，就顺序的从数栈和符号栈中pop出相应的数和符号，并运行
        while (true) {
            //如果符号栈为空，则计算到最后的结果，数栈中只有一个数字
            if (operStack.isEmpty()) {
                break;
            }
            num1 = numStack.pop();
            num2 = numStack.pop();
            oper = operStack.pop();
            res = numStack.cal(num1, num2, oper);
            numStack.push(res);//入栈
        }
        System.out.printf("表达式%s = %d", expression, numStack.pop());


    }
}

class ArrayStack2 {
    private int maxSize;//栈的大小
    private int[] stack;//数组，数组模拟栈，数据就放在该数组
    private int top = -1;

    //构造器
    public ArrayStack2(int maxSize) {
        this.maxSize = maxSize;
        stack = new int[maxSize];
    }

    //增加一个方法，可以返回当前栈顶的值，但不是真正的pop
    public int peek() {
        return stack[top];
    }

    //栈满
    public boolean isFull() {
        return top == maxSize - 1;
    }

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

    //入栈 -push
    public void push(int value) {
        //先判断栈是否满
        if (isFull()) {
            System.out.println("栈满");
            return;
        }
        top++;
        stack[top] = value;
    }

    //出栈 -pop，将栈顶的数据返回
    public int pop() {
        if (isEmpty()) {
            throw new RuntimeException("栈空没有数据");
        }
        int value = stack[top];
        top--;
        return value;
    }

    //显示栈的情况【遍历栈】，遍历时从栈顶开始显示数据
    public void list() {
        if (isEmpty()) {
            System.out.println("栈空，没有数据");
            return;
        }
        for (int i = top; i >= 0; i--) {
            System.out.printf("stack[%d]=%d\n", i, stack[i]);
        }
    }

    //返回运算符的优先级，优先级使用数字表示
    //数字越大，则优先级就越高
    public int priority(int oper) {
        if (oper == '*' || oper == '/') {
            return 1;
        } else if (oper == '+' || oper == '-') {
            return 0;
        } else {
            return -1;
        }
    }

    //判断是不是一个运算符
    public boolean isOper(char val) {
        return val == '+' || val == '-' || val == '*' || val == '/';
    }

    //计算方法
    public int cal(int num1, int num2, int oper) {
        int res = 0;//用于存放计算的结果
        switch (oper) {
            case '+':
                res = num1 + num2;
                break;
            case '-':
                res = num2 - num1;
                break;
            case '*':
                res = num1 * num2;
                break;
            case '/':
                res = num2 / num1;
                break;
            default:
                break;
        }
        return res;
    }

}

输入 ==70+2*6-2==

输出

图片.png