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);
        String s = in.nextLine();
        //先把所有括号统一
        s = s.replace("{","(");
        s = s.replace("[","(");
        s = s.replace("}",")");
        s = s.replace("]",")");
        //初始字符串预处理（提取数字整体作为一个字符串，创建字符串数组，其中包含对负数的处理）
        List<String> list = String2List(s);
        //中缀表达式转后缀表达式
        List<String> ss = mid2back(list);
        //后缀表达式计算
        System.out.print(backCal(ss));
    }
    public static List<String> mid2back(List<String> s){
        List<String> ss = new ArrayList<>();
        Stack<String> stack = new Stack<String>();
        for(int i=0; i<s.size(); i++){
            String c = s.get(i);
            //System.out.println(c);
            //数字直接输出，字符判断入栈(请注意：左括号的优先级，入栈后视为最低)
            if(isNum(c)){
                ss.add(c);
            }else{
                if(c.equals("(") || stack.isEmpty()){
                    stack.push(c);
                }
                else if(c.equals(")")){
                    //出栈直到碰到左括号
                    while(!stack.peek().equals("(")){
                        ss.add(stack.pop());
                    }
                    //左括号弹出去
                    stack.pop();
                }
                else if(c.equals("*") || c.equals("/")){
                    //栈顶运算符优先级低于当前操作符，入栈
                    if(stack.peek().equals("+") || stack.peek().equals("-") || stack.peek().equals("(") ){
                        stack.push(c);
                    }else{
                        //出栈，直到栈顶优先级低于当前操作符，或者栈空，然后入栈
                        while(!stack.isEmpty() && (stack.peek().equals("*") || stack.peek().equals("/"))){
                            ss.add(stack.pop());
                        }
                        stack.push(c);
                    }
                }
                else if(c.equals("+") || c.equals("-")){
                    if(stack.peek() == "("){
                        stack.push(c);
                    }else{
                        while(!stack.isEmpty() && !stack.peek().equals("(") ){
                            ss.add(stack.pop());
                        }
                        stack.push(c);
                    }
                    
                }
            }
        }
        //如果栈内还有运算符，直接弹出，直到栈空
        while(!stack.isEmpty()){
            ss.add(stack.pop());
        }
        //System.out.println(ss);
        return ss;
    }
    public static String backCal(List<String> ss){
        //这里必须为String类型，不然后边字符和数字的转换会出现问题
        Stack<String> stack = new Stack<>();
        for(int i=0; i<ss.size(); i++){
            String c = ss.get(i);
            //数字直接入栈
            if(isNum(c)){
                stack.push(c);
            }
            //运算符，取出来并计算，再放进去
            else{
                Long b = Long.parseLong(stack.pop());
                Long a = Long.parseLong(stack.pop());
                switch(c){
                    case "+":
                        stack.push(Long.toString(a + b));
                        break;
                    case "-":
                        stack.push(Long.toString(a - b));
                        break;
                    case "*":
                        stack.push(Long.toString(a * b));
                        break;
                    case "/":
                        stack.push(Long.toString(a / b));
                        break;
                }
            }
        }
        return stack.pop();
    }
    public static boolean isNum(String s){
        for(int i = 0; i <= 9; i++){
            if(s.contains(i+"")){
                return true;
            }
        }
        return false;
    }
    public static List<String> String2List(String s){
        List<String> list = new ArrayList<>();
        String temp = "";
        for(int i=0; i<s.length();i++){
            char c = s.charAt(i);
            if(Character.isDigit(c)){
                if(i!=s.length()-1 && Character.isDigit(s.charAt(i+1))){
                    temp += c;
                }else{
                    temp += c;
                    list.add(temp);
                    temp = "";
                }
            }else if(c == '-'){
                //对负数特殊处理(如“-4”要当作一个操作数来处理)
                if(i == 0 || s.charAt(i-1) == '('){
                    temp += c;
                }else{
                    list.add(c + "");
                }
            }else{
                list.add(c + "");
            }
        }
        return list;
    }
}