用Java代码写出输入表达式就可以求出具体值的代码，用栈或者队列的形式

Question

用Java代码写出输入表达式就可以求出具体值的代码，用栈或者队列的形式

Answer 1

public class Stack<T> {
    //节点类
    public class Node{
        public T data;
        public Node next;
        public Node(){}
        public Node(T data,Node next){
            this.data = data;
            this.next = next;
        }
    }//Node
    
    public Node top = new Node();
    public int size;
    public Node oldNode;
    
    //入栈
    public void push(T element){
        top = new Node(element,top);
        size++;
    }
    
    //出栈
    public T pop(){
        oldNode = top;
        top = top.next;
        //oldNode = null;
        size--;
        return oldNode.data;
    }
    
    //返回栈顶对象的数据域，但不出栈
    public T peek(){
        return top.data;
    }
    
    //栈长
    public int length(){
        return size;
    }
    
    //判断栈是否为空
    public boolean isEmpty(){
        return size == 0;
    }

}

import java.util.Scanner;
public class Test {
    public static final char[][] relation = {{'>','>','<','<','<','>','>'},
            {'>','>','<','<','<','>','>'},{'>','>','>','>','<','>','>'},
            {'>','>','>','>','<','>','>'},{'<','<','<','<','<','=','!'},
            {'>','>','>','>','!','>','>'},{'<','<','<','<','<','!','='}};
    
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        while(true){
        try{
        System.out.println("请输入要计算的表达式：");
        String exp = input.next();
        System.out.println(calc(exp + "#"));
        }catch(ArithmeticException e1){
            System.out.println("表达式中的分母不能为0");
            e1.printStackTrace();
        }
        }
    }
    /**
     * 
     * @param exp 要计算的表达式
     * @return 计算的结果
     */
    private static int calc(String exp) {
        //操作数栈
        Stack<Integer> num = new Stack<Integer>();
        //操作符栈
        Stack<Character> op = new Stack<Character>();
        
        op.push('#');
        int i = 0;
        char ch = exp.charAt(i);
        boolean flag = false;//判断连续的几个字符是否是数字，若是，就处理成一个数字。这样就能处理多位数的运算了。
        while(ch != '#' || op.peek() != '#') {//peek()查看栈顶对象但不移除。
            if(ch >= '0' && ch <= '9') {
                if(flag) {
                    int tmp = num.pop();
                    num.push(tmp * 10 + Integer.parseInt(ch + ""));
                } else {
                    num.push(Integer.parseInt(ch + ""));
                }
                flag = true;
                i++;
            } else {
                flag = false;
                switch(precede(op.peek(), ch)) {
                case '<':
                    op.push(ch);
                    i++;
                    break;
                case '=':
                    op.pop();
                    i++;
                    break;
                case '>':
                    int num2 = num.pop();
                    int num1 = num.pop();
                    int result = operate(num1, op.pop(), num2);
                    num.push(result);
                    break;
                case '!':
                    System.out.println("输入的表达式错误！");
                    return -1;
                }
            }
            ch = exp.charAt(i);
        }
        return num.peek();
    }
    private static char precede(char peek, char ch) {
        return relation[getIndex(peek)][getIndex(ch)];
    }
    
    /**
     * 
     * @param ch 操作符
     * @return 操作符的索引，按照+、-、*、/、（、）的顺序
     */
    private static int getIndex(char ch) {
        int index = -1;
        switch(ch) {
        case '+':
            index = 0;
            break;
        case '-':
            index = 1;
            break;
        case '*':
            index = 2;
            break;
        case '/':
            index = 3;
            break;
        case '(':
            index = 4;
            break;
        case ')':
            index = 5;
            break;
        case '#':
            index = 6;
            break;
        }
        return index;
    }
    
    /**
     * 
     * @param num1  第一个运算数
     * @param ch 运算符
     * @param num2 第二个运算数
     * @return 运算结果
     */
    private static int operate(int num1, char ch, int num2) {
        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;
    }
}