某大神的，理解后敲出来。题解 | #四则运算#

import java.util.Scanner;
import java.util.Stack;

// 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) {
        Scanner scanner = new Scanner(System.in);
        scanner.useDelimiter("\n");
        while (scanner.hasNext()) {
            String S = scanner.next();
            S = S.replaceAll("\\[", "(").replaceAll("]", ")")
                .replaceAll("\\{", "(").replaceAll("}", ")");
            System.out.println(yunsuan(S));
        }
    }

    public static int yunsuan(String S) {
        char opt = '+';
        int length = S.length();
        Stack<Integer> stack = new Stack<>();
        for (int i = 0; i < S.length(); i++) {
            char c = S.charAt(i);
            if (Character.isDigit(c)) {
                int num = 0;
                while (i < length && Character.isDigit(S.charAt(i))) {
                    num = num * 10 + S.charAt(i) - 48;
                    i++;
                }
                i--;
                if (opt == '+') stack.push(num);
                else if (opt == '-') stack.push(-num);
                else if (opt == '*') stack.push(stack.pop() * num);
                else if (opt == '/') stack.push(stack.pop() / num);
            } else if (c == '+' || c == '-' || c == '*' || c == '/') {
                opt = c;
            } else if (c == '(') {
                int left = i;
                int right = i + 1;
                int count = 1;
                while (right < length && count > 0) {
                    if (S.charAt(right) == '(') count++;
                    if (S.charAt(right) == ')') count--;
                    right++;
                }
                i = right - 1;
                int num = yunsuan(S.substring(left + 1, right - 1));
                if (opt == '+') stack.push(num);
                else if (opt == '-') stack.push(-num);
                else if (opt == '*') stack.push(stack.pop() * num);
                else if (opt == '/') stack.push(stack.pop() / num);
            }
        }
        int res = 0;
        while (!stack.isEmpty()) {
            res += stack.pop();
        }
        return res;
    }
}