题解 | #表达式求值#

import java.util.*;

// 这道题不应该是简单题啊
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String s = in.nextLine();
        int num1 = 0;
        int o1 = 1;
        int num2 = 1;
        int o2 = 1;
        Stack<Integer> stk = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                int cur = 0;
                while (i < s.length() && Character.isDigit(s.charAt(i))) {
                    cur = cur * 10 + (s.charAt(i) - '0');
                    i++;
                }
                i--;
                num2 = o2 == 1 ? num2 * cur : num2 / cur;
            } else if (c == '*' || c == '/') {
                o2 = c == '*' ? 1 : -1;
            } else if (c == '(') {//遇到括号保存当前结果，并初始化
                stk.push(num1);
                stk.push(o1);
                stk.push(num2);
                stk.push(o2);

                num1 = 0;
                o1 = 1;
                num2 = 1;
                o2 = 1;
            } else if (c == '+' ||
                       c == '-') {//遇到加减，说明可以开始计算，计算num1并定义其他几个变量
                if (c == '-' && (i == 0 || s.charAt(i - 1) == '(')) {
                    o1 = -1;
                    continue;
                }
                num1 += o1 * num2;
                o1 = (c == '+' ? 1 : -1);
                num2 = 1;
                o2 = 1;
            } else {//遇到右括号，则出栈，并计算num2
                int cur = num1 + o1 * num2;
                o2 = stk.pop();
                num2 = stk.pop();
                o1 = stk.pop();
                num1 = stk.pop();

                num2 = o2 == 1 ? num2 * cur : num2 / cur;
            }

        }
        System.out.println(num1 + o1 * num2);
    }
}