算法基础07：栈及实现简单计算器

栈的实际需求

栈的介绍

应用场景

代码实现

快速入门

思路分析

代码

package com.algorithm.stack;

import java.util.Scanner;

/**
 * TODO
 *
 * @author：XMD
 * @date: 23.8.22
 */
public class ArrayStackDemo {

    public static void main(String[] args) {
        ArrayStack arrayStack = new ArrayStack(5);
        String key = "";
        boolean loop = true;
        Scanner scanner = new Scanner(System.in);

        while (loop) {
            key = scanner.next();
            switch (key) {
                case "show":
                    arrayStack.show();
                    break;
                case "push":
                    System.out.println("输入");
                    int value = scanner.nextInt();
                    arrayStack.push(value);
                    break;
                case "pop":
                    try {
                        int res = arrayStack.pop();
                        System.out.println("pop:" + res);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    break;
                case "exit":
                    scanner.close();
                    loop = false;
                    break;
            }

            System.out.println("结束");
        }
    }

}

class ArrayStack {
    //栈的大小
    private int maxSize;
    //数组，模拟栈，存放数据
    private int[] stack;
    //栈顶，默认-1
    private int top = -1;

    public ArrayStack(int maxSize) {
        this.maxSize = maxSize;
        stack = new int[this.maxSize];
    }

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

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

    public void push(int value) {
        if (isFull()){
            System.out.println("栈满");
            return;
        }
        top++;
        stack[top] = value;
    }

    public int pop() {
        if (isEmpty()) throw new RuntimeException("栈空");

        int value = stack[top];
        top--;
        return value;
    }

    public void show() {
        if (isEmpty()) {
            System.out.println("栈空");
        }
        for (int i = top; i >= 0; i--) {
            System.out.println(stack[i]);
        }
    }
}

实现综合计算器

思路

代码实现

package com.algorithm.stack;

/**
 * 栈实现综合计算器 （单位数的加减乘除）
 *
 * @author：XMD
 * @date: 23.8.22
 */
public class Calculator {
    public static void main(String[] args) {
        String exp = "70+2*60-4";
        ArrayStack2 numStack = new ArrayStack2(10);
        ArrayStack2 operStack = new ArrayStack2(10);
        //定义需要的变量

        //用于扫描
        int index = 0;
        int num1;
        int num2;
        int oper;
        int res;
        //将每次扫描得到的char保存到ch
        char ch = ' ';
        String keepNum = "";
        while (true) {
            ch = exp.substring(index, index + 1).charAt(0);
            //如果是运算法
            if (operStack.isOper(ch)) {
                //判断符号栈是否为空
                if (operStack.isEmpty()) {
                    //直接入符合栈
                    operStack.push(ch);
                } else {
                    //进行比较，如果当前操作符优先级小于等于女栈中的优先级，就需要从栈中pop出两个数
                    //再从符号栈中pop出一个符号，进行运算，将得到的结果，入数栈，然后将当前的操作符入符号栈
                    if (operStack.priority(ch) <= operStack.priority(operStack.peek())) {
                        num1 = numStack.pop();
                        num2 = numStack.pop();
                        oper = operStack.pop();
                        res = numStack.cal(num1, num2, oper);
                        numStack.push(res);
                        operStack.push(ch);
                    } else {
                        //如果当前操作符的优先级大于栈中的操作符，就直接入符号栈
                        //为什么要减48，需要看ASCII表，十进制
                        operStack.push(ch);
                    }
                }
            } else {
                //如果是数，直接入数栈
               // numStack.push(ch - 48);
                //思路分析
                //1.当处理多为数是，不能发现是一个数就立即入栈，因为可能是多位数
                //2.在处理数时。需要向exp的表达式的index后再看一位，如果是数就进行扫描，如果是符号就入栈
                //3.因此需要定义一个变量字符串keepNum，用于拼接
                keepNum += ch;

                //如果ch是最后一位，就直接入栈
                if (index == exp.length() -1){
                    numStack.push(Integer.parseInt(keepNum));
                }else {
                    //判断下一个字符是不是数字，如果是数字就继续扫描，如果是运算符，就入栈
                    //注意只是看后一位，不是出栈
                    if (operStack.isOper(exp.substring(index+1,index+2).charAt(0))){
                        //如果后一位是运算符，就入栈
                        numStack.push(Integer.parseInt(keepNum));
                        //keepNum清空
                        keepNum = "";
                    }
                }
            }

            //让index + 1 ，并判断是否扫描到exp最后
            index++;
            if (index >= exp.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，数栈中只有一个数组，就是结果
            if (operStack.isEmpty()) {
                break;
            }
            num1 = numStack.pop();
            num2 = numStack.pop();
            oper = operStack.pop();
            res = numStack.cal(num1, num2, oper);
            numStack.push(res);
        }
        System.out.println("结果是：" + numStack.pop());

    }
}

class ArrayStack2 {
    //栈的大小
    private int maxSize;
    //数组，模拟栈，存放数据
    private int[] stack;
    //栈顶，默认-1
    private int top = -1;

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

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

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

    public void push(int value) {
        if (isFull()) {
            System.out.println("栈满");
            return;
        }
        top++;
        stack[top] = value;
    }

    public int pop() {
        if (isEmpty()) throw new RuntimeException("栈空");

        int value = stack[top];
        top--;
        return value;
    }

    //返回栈顶数据，不出栈
    public int peek() {
        if (isEmpty()) throw new RuntimeException("栈空");
        return stack[top];
    }

    public void show() {
        if (isEmpty()) {
            System.out.println("栈空");
        }
        for (int i = top; i >= 0; i--) {
            System.out.println(stack[i]);
        }
    }

    //返回运算符的优先级，优先级是程序员来确定，优先级使用数字表示
    //数字越大，优先级越高
    //假定表达式只有 + - * /
    public int priority(int oper) {
        if (oper == '*' || oper == '/') {
            return 1;
        } else if (oper == '+' || oper == '-') {
            return 0;
        } else {
            return -1;
        }
    }

    //判断是否是运算符
    public boolean isOper(char val) {
        return val == '+' || val == '-' || val == '*' || val == '/';
    }

    //计算方法
    public int cal(int num1, int num2, int oper) {
        //计算结果
        int res = 0;
        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;
    }
}