使用「双栈」解决「究极表达式计算」问题

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.println(calculate(sc.nextLine()));
    }

    private static Map<Character, Integer> map = new HashMap() {{
        put('+', 1);
        put('-', 1);
        put('*', 2);
        put('/', 2);
        put('%', 2);
        put('^', 3);
    }};

    public static int calculate(String s) {
        //s预处理
        s = s.replaceAll(" ", "");
        s = s.replaceAll("\\(-", "(0-");
        char[] chs = s.toCharArray();
        //定义双栈
        Deque<Integer> nums = new ArrayDeque<>();
        nums.addLast(0);
        Deque<Character> opts = new ArrayDeque<>();
        //开始模拟计算
        for (int i = 0; i < chs.length; i++) {
            char ch = chs[i];
            if (ch == '(') {
                //左括号处理：直接放入opts栈中
                opts.addLast(ch);
            } else if (ch == ')') {
                //右括号处理:计算到最近的左括号
                while (!opts.isEmpty()) {
                    if (opts.peekLast() != '(') {
                        calculate(nums, opts);
                    } else {
                        opts.pollLast();
                        break;
                    }
                }
            } else if (Character.isDigit(ch)) {
                //数字处理:取出[完整]数字放入nums栈中
                int num = 0;
                while (i < chs.length && Character.isDigit(chs[i]))   num = num * 10 + (chs[i++] - '0');
                i--;
                nums.addLast(num);
            } else {
                //运算符号处理:处理opts栈顶元素比当前字符优先级高的元素
                while (!opts.isEmpty() && opts.peekLast() != '(' && map.get(opts.peekLast()) >= map.get(ch)) {
                    calculate(nums, opts);
                }
                opts.addLast(ch);
            }
        }
        //处理剩余计算
        while (!opts.isEmpty()) calculate(nums, opts);
        //返回计算结果
        return nums.pollLast();


    }

    private static void calculate(Deque<Integer> nums, Deque<Character> opts) {
        int a = nums.pollLast(), b = nums.pollLast(), ans = 0;
        char opt = opts.pollLast();
        switch (opt) {
            case '+':
                ans = b + a;
                break;
            case '-':
                ans = b - a;
                break;
            case '*':
                ans = b * a;
                break;
            case '/':
                ans = b / a;
                break;
            case '%':
                ans = b % a;
                break;
            case '^':
                ans = (int) Math.pow(b, a);
        }
        nums.addLast(ans);
    }

}