题解 | #四则运算#

import java.util.*;

// 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 in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNext()) { // 注意 while 处理多个 case
            String exp = in.next();
            //括号替换
            String s = exp.replaceAll("\\[", "(");
            String s1 = s.replaceAll("\\{", "(");
            String s2 = s1.replaceAll("\\}", ")");
            String s3 = s2.replaceAll("\\]", ")");
            //System.out.println(s3);
            int length = s3.length();
            //操作数和运算符
            Stack<Character> op = new Stack<>();
            Stack<Integer> data = new Stack<>();
            for (int i = 0; i < length; i++) {
                char ch = s3.charAt(i);
                //操作数
                if (Character.isDigit(ch)) {
                    //提取数字
                    int index = i;
                    while (index != length-1 && Character.isDigit(s3.charAt(index))) {
                        index++;
                    }
                    String temp = "";
                    if (i != length - 1) {
                        temp = s3.substring(i, index);
                    } else {
                        temp = s3.substring(index, length);
                    }
                    int num = Integer.parseInt(temp);
                    data.push(num);
                    //下标跳转
                    if (i != length - 1) {
                        i = index - 1;
                    }
                    continue;
                }
                //运算符
                if (op.isEmpty() || ch == '(') {
                    //如果是正负号，在前面加一个0
                    if (ch == '+' || ch == '-') {
                        if (i - 1 < 0 || s3.charAt(i - 1) == '(') {
                            data.push(0);
                        }
                    }
                    //符号入栈
                    op.push(ch);
                } else if (ch == ')') {
                    //进行计算
                    while (op.peek() != '(') {
                        compute(op, data);
                    }
                    //左括号出栈
                    char wasted = op.pop();
                } else if (ch == '+' || ch == '-') {
                    //检查优先级:真不为空且栈顶元素不为左括号，一直进行出栈运算
                    while (!op.isEmpty() && op.peek() != '(') {
                        compute(op, data);
                    }
                    //正负号前加0
                    if (i - 1 < 0 || s3.charAt(i - 1) == '(') {
                        data.push(0);
                    }
                    op.push(ch);
                } else if (ch == '*' || ch == '/') {
                    //优先级检查
                    while (!op.isEmpty() &&op.peek() != '('&& op.peek() != '+' && op.peek() != '-') {
                        compute(op, data);
                    }
                    op.push(ch);
                }
            }
            //继续计算
            while (!op.isEmpty()) {
                compute(op, data);
            }
            System.out.print(data.pop());
        }
    }
    public static void compute(Stack<Character> op, Stack<Integer> data) {
        char operator=op.pop();
        int right=data.pop();
        int left=data.pop();
        //结果
        int res=-1;
        if(operator=='+'){
            res=left+right;
        }else if(operator=='-'){
            res=left-right;
        }else if(operator=='*'){
            res=left*right;
        }else{
            res=res=left/right;
        }
        data.push(res);
    }
}