package A;

public class JosePhu {
    public static void main(String[] args) {
        String expression="300+2*6-2";
        //你可以画两个表示栈的图
        ArrayStack numStack=new ArrayStack(10);//数字栈
        ArrayStack operStack=new ArrayStack(10);//符号栈
        int index=0;
        int num1=0;
        int num2=0;
        int oper=0;
        int res=0;
        char ch=‘ ‘;
        String keepNum="";
        while (true){
            //substring能够截取字符串的某一段，取一个之后，它本质上还是一个字符串，所以利用charAt(0)，取一个char类型的字符
            ch=expression.substring(index,index+1).charAt(0);
            if (operStack.isOper(ch)){
                if (!operStack.isEmpty()){//这是判断这个栈是不是空的
                    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 {
                        operStack.push(ch);
                    }
                }else {
                    operStack.push(ch);
                }
            }else {
                /*
                * 就是说这里就是解决一个1+2和10+2的问题，因为它每次只探了一位*/
                keepNum=keepNum+ch;
                //如果是最后一位，直接压进去就可以了
                if (index==expression.length()-1){
                    numStack.push(Integer.parseInt(keepNum));
                }else {
                    //就是说再往后面探一位
                    if (operStack.isOper(expression.substring(index+1,index+2).charAt(0))){
                        //将字符串转为int类型的方法，比如"123"->123
                        numStack.push(Integer.parseInt(keepNum));
                        keepNum="";//使用完要将keepNum清空
                    }
                }
            }
            index++;
            if (index>=expression.length()){
                break;
            }
        }
        while (true){
            if (operStack.isEmpty()){
                break;
            }
            num1=numStack.pop();
            num2=numStack.pop();
            oper=operStack.pop();
            res=numStack.cal(num1,num2,oper);
            numStack.push(res);
        }
        int res2=numStack.pop();
        System.out.println("表达式"+expression+"="+res2);
    }
}

package A;

public class ArrayStack {
    private int[] stack;
    private int maxSize;
    private int top=-1;

    public ArrayStack(int maxSize) {
        this.maxSize = maxSize;
        stack=new int[this.maxSize];
    }
    public int peek(){//当前位置的栈顶
        return stack[top];
    }
    public boolean isFull(){
        return top==maxSize-1;//按数组0开始，那种思路来考虑的
    }
    public boolean isEmpty(){
        return top==-1;
    }
    public void push(int value){
        if (isFull()){
            System.out.println("本栈已满，");
            return;
        }else {
            top++;
            stack[top]=value;
        }
    }
    public int pop(){
        if (isEmpty()){
            System.out.println("本栈已为空，");
            return 0;
        }else {
            int value=stack[top];
            top--;
            System.out.println("此次出栈的数为："+value);
            //注意，例如‘*’的ASCII=42，所以它就出了42
            return value;
        }
    }

    public int priority(int oper){
        if (oper==‘*‘||oper==‘/‘){
            return 1;
        }else if (oper==‘+‘||oper==‘-‘){
            return 0;
        }else {
            return -1;
        }
    }
    public boolean isOper(char val){
        return val==‘+‘||val==‘-‘||val==‘*‘||val==‘/‘;
    }
    public int cal(int num1,int num2,int oper){
        int res=0;
        switch (oper){
            case ‘+‘:
                res=num1+num2;
                break;
            case ‘-‘:
                //由主函数中的while得知，num2是栈底，比如3-2中，3就是num2
                res=num2-num1;
                break;
            case ‘*‘:
                res=num1*num2;
                break;
            case ‘/‘:
                res=num2/num1;
                break;
            default:
                break;
        }
        return res;
    }
}

用栈来实现计算器