题解 | #表达式求值#

import java.util.*;
import java.io.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        try {
            String s = br.readLine();
            s = s.replaceAll("\\{", "(");
            s = s.replaceAll("\\}", ")");
            s = s.replaceAll("\\[", "(");
            s = s.replaceAll("\\]", ")");
            System.out.println(resolve(s));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    public static int resolve(String str) {
        char sign = '+';
        int num = 0;
        Stack<Integer> stack = new Stack();
        int length = str.length();
        for (int i = 0; i < length; i++) {
            char c = str.charAt(i);
            if (c == ' ')continue; //注意点3，需要处理空字符
            if (Character.isDigit(c)) {
                //num = c-'0';//注意点2，若遇到大于9的数，这个地方没有处理
                num = 10 * num + c -'0'; 
            } else if (c == '(') {
                int j = i + 1;
                //注意点4，无法处理括号嵌套{[()]}，首先需要在程序最开始将所有{}[]替换为()
                //其次需要在这里找出最外层左括号对应的最外层右括号
                /* while (str.charAt(j)!=')'){
                     j++;
                 }*/
                int count = 1;
                while (count > 0) {
                    if (str.charAt(j) == ')') {
                        count--;
                    } else if (str.charAt(j) == '(') {
                        count++;
                    }
                    j++;
                }
                num = resolve(str.substring(i + 1, j - 1));
                i = j - 1; //注意点1，需要更新i的索引为右括号的索引，这样可以保证括号内的计算结果在下边可以加入栈
            }
            if (!Character.isDigit(c) || i == length - 1) {
                switch (sign) {
                    case '+':
                        stack.push(num);
                        break;
                    case '-':
                        stack.push(num * (-1));
                        break;
                    case '*':
                        stack.push(stack.pop() * num);
                        break;
                    case '/':
                        stack.push(stack.pop() / num);
                        break;
                }
                num = 0;
                sign = c;
            }
        }
        int res = 0;
        while (!stack.isEmpty()) {
            res += stack.pop();
        }
        return res;
    }
}