Dijkstra的双栈算术表达式求值算法

package First;

import java.util.Scanner;
import java.util.Stack;
import java.util.regex.Pattern;

public class Evaluate {
    public static void main(String[] args) {
        //运算符栈
        Stack opsStack = new Stack<>();
        //操作数栈
        Stack valsStack = new Stack<>();
        //定义需要压入运算符栈的运算符
        String ops = "+-*/sqrt";
        Scanner input = new Scanner(System.in);
        //接收表达式
        String exp = input.nextLine();
        for(int i = 0;i < exp.length();i++){
            String s = exp.charAt(i) + "";
            if(s.matches("[\\(\\s]")) continue;
            //压入运算符栈
            if (ops.contains(s)){
                opsStack.push(s);
            } else if(s.equals(")")){ //如果字符为”）“，弹出运算符与操作数，计算结果并压入操作数栈
                String op = opsStack.pop();
                double v = valsStack.pop();
                if(op.equals("+")) v = valsStack.pop() + v;
                else if(op.equals("-")) v = valsStack.pop() - v;
                else if(op.equals("*")) v = valsStack.pop() * v;
                else if(op.equals("/")) v = valsStack.pop() / v;
                else if(op.equals("sqrt")) v = Math.sqrt(v);
                valsStack.push(v);
            }
            //如果字符既非运算符也不是括号，将它作为double值压入栈
            else valsStack.push(Double.parseDouble(s));
        }
        System.out.println(valsStack.pop());
    }
}