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 {
    private static int i = 0;//i作工作指针

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNextLine()) { // 注意 while 处理多个 case
            char[] ch = in.nextLine().toCharArray();
            System.out.println(compute(ch));
            i = 0;//每处理一个字符串更新指针
        }
    }

    private static int compute(char[] ch) {
        Stack<Integer> stack = new Stack<>();
        char sign = '+';
        int len = ch.length;
        int num = 0;
        while (i < len) {
            //遇左括号递归
            if (ch[i] == '(') {
                i++;
                num = compute(ch);
            }

            //遇数字计算
            while (i < len && Character.isDigit(ch[i])) {
                num = num * 10 + (ch[i] - '0');
                i++;
            }

            //遇到符号才对栈处理
            if (sign == '+')stack.push(num);
            if (sign == '-')stack.push(-num);
            if (sign == '*')stack.push(num * stack.pop());
            if (sign == '/')stack.push(num / stack.pop());

            if (i >= ch.length)break; //最后一次递归可能越界

            sign = ch[i]; //更新符号
            num = 0;

            if (ch[i] == ')') {
                i++;
                break;
            }
            i++;//没有碰到回括号，指针后移
        }
        int sum = 0;
        while (stack.size() != 0) {
            sum += stack.pop();
        }
        return sum;

    }
}