题解 | #四则运算#

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine();
        int res = calculate(str);
        System.out.println(res);
    }
  
    public static int calculate(String s) {
        s = s.replaceAll("\\{", "(");
        s = s.replaceAll("\\}", ")");
        s = s.replaceAll("\\[", "(");
        s = s.replaceAll("\\]", ")");
        s = s.replaceAll("--", "+");
        s = s.replaceAll("\\+-", "-");
        s = s.replaceAll("-\\+", "-");
        s = s.replaceAll("\\+\\+", "+");
        int start = -1, end = -1;
        if (s.contains("+") || s.contains("-") || s.contains("*") || s.contains("/") ||
                s.contains("(") || s.contains(")")) {
            //计算小括号,先找出第一个左括号(的位置,再找出其对应的右括号)
            //然后截取括号中间部分的字符串进行递归运算，将所得结果再插入原式替代原来的括号
            start = s.indexOf("(");
            int temp = 0;
            for (int i = start + 1; i < s.length(); i++) {
                if (s.charAt(i) == '(') {
                    temp++;
                }
                if (s.charAt(i) == ')') {
                    if (temp == 0) {
                        end = i;
                        break;
                    } else {
                        temp--;
                    }
                }
            }
            if (start != -1 && end != -1) {
                int res = calculate(s.substring(start + 1, end));
                return calculate(s.substring(0,start) + String.valueOf(res) + s.substring(end + 1));
            }

            //计算乘法,确定乘号*左右两边的乘数在字符串中的位置，再切割出乘数a、b
            int x = s.indexOf("*");
            if (x != -1) {
                int m = x - 1, n = x + 1;
                if (s.charAt(x + 1) == '-' || s.charAt(x + 1) == '+') n = x + 2;
                while (m >= 0 && Character.isDigit(s.charAt(m))) {
                    m--;
                }
                while (n < s.length() && Character.isDigit(s.charAt(n))) {
                    n++;
                }
                int a = Integer.parseInt(s.substring(m + 1, x));
                int b = Integer.parseInt(s.substring(x + 1, n));
                int res = a * b;
                return calculate(s.substring(0, m + 1) + String.valueOf(res) + s.substring(n));
            }

            //计算除法,确定除号/左右两边的除数与被除数在字符串中的位置，再切割出除数与被除数a、b
            int y = s.indexOf("/");
            if (y != -1) {
                int m = y - 1, n = y + 1;
                if (s.charAt(y + 1) == '-' || s.charAt(y + 1) == '+') n = y + 2;
                while (m >= 0 && Character.isDigit(s.charAt(m))) {
                    m--;
                }
                while (n < s.length() && Character.isDigit(s.charAt(n))) {
                    n++;
                }
                int a = Integer.parseInt(s.substring(m + 1, y));
                int b = Integer.parseInt(s.substring(y + 1, n));
                int res = a / b;
                return calculate(s.substring(0, m + 1) + String.valueOf(res) + s.substring(n));

            }

            //计算加减法,分别用正负号和数字分割字符串,得出数字数组与符号数组,以便进行计算
            //应分为两种情况：2+3(数字在前)、-2-3(正负号在前)
            int sum;
            String[] digitArr = s.split("[+-]");
            String[] charArr = s.split("[0-9]+");
            if (s.charAt(0) == '+' || s.charAt(0) == '-') {
                sum = 0;
                for (int i = 0; i < charArr.length; i++) {
                    if ("-".equals(charArr[i])) {
                        sum = sum - Integer.parseInt(digitArr[i + 1]);
                    } else if ("+".equals(charArr[i])) {
                        sum = sum + Integer.parseInt(digitArr[i + 1]);
                    }
                }
            } else {
                sum = Integer.parseInt(digitArr[0]);
                for (int i = 1; i < charArr.length; i++) {
                    if ("-".equals(charArr[i])) {
                        sum = sum - Integer.parseInt(digitArr[i]);
                    } else if ("+".equals(charArr[i])) {
                        sum = sum + Integer.parseInt(digitArr[i]);
                    }
                }
            }
            return sum;
        }
        //若没有符号,只有数字,则直接转为数字返回
        return Integer.parseInt(s);
    }
}