题解 | #表达式求值#

import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {

    static Deque<Integer> num = new ArrayDeque<>();
    static Deque<Character> op = new ArrayDeque<>();
    static Map<Character, Integer> pr = new HashMap<>();

    public static boolean digit(char c) {
        return c >= '0' && c <= '9';
    }

    public static void compute() {
        int b = num.pop(); // 第一个出栈的是第二个操作数
        int a = num.pop();
        char c = op.pop();

        int x = 0;
        if (c == '+') x = a + b;
        else if (c == '-') x = a - b;
        else if (c == '*') x = a * b;
        else x = a / b;
        num.push(x);
    }

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

        pr.put('+', 1);
        pr.put('-', 1);
        pr.put('*', 2);
        pr.put('/', 2);

        char lastch = ' ';
        char symbol = '+';
        for (int i = 0; i < exp.length(); i ++) {
            char c = exp.charAt(i);
            if (digit(c)) {
                int x = 0, j = i;
                while (j < exp.length() && digit(exp.charAt(j))) 
                    x = x * 10 + exp.charAt(j ++) - '0';
                i = j - 1;
                if (symbol == '-') x = x * (-1);
                num.push(x);
            } 
            else if (c == '(') op.push(c);
            else if (c == ')') {
                while(op.peek() != '(') compute();
                op.pop();
            }
            else {
                if (c == '-' && !digit(lastch) && lastch != ')') symbol = '-';
                else {
                    symbol = '+';
                    while (op.size() != 0 && op.peek() != '(' && 
                    pr.get(op.peek()) >= pr.get(c)) compute();
                    op.push(c);
                }
            }
            lastch = c;
        }

        while(op.size() != 0) compute();
        System.out.println(num.peek());
    }

}