题解 | #表达式求值#

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

public class Main{
    public static void main(String [] args) throws Exception{
        BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
        String str = bf.readLine();
        int x = 0;
        System.out.print(calc(str));
        
    }
    public static int calc(String str){
        Stack<Integer> stack=new Stack<>();
        int n = str.length();
        char [] arr = str.toCharArray();
        int num =0;
        char sign='+';//定义字符串,第一次的时候一定是+;
        for(int i=0;i<n;i++){
            char x = arr[i];
            if(x ==' ')continue;//空格 继续
            if(Character.isDigit(x)){
               num =10*num + x-'0';//纯数字,一直累加
            }
            if(x == '('){//当遇到左括号时,把这一部分拿出来计算
               int j = i+1;
               int count=1;
               while(count>0){//括号 对等
                    if(arr[j]=='(')count++;
                    if(arr[j]==')')count--;
                    j++;
               }
               num=calc(str.substring(i+1,j-1));//递归计算括号里里面的值
               i=j-1;//while中多加了一次,待会还要执行i++,所以这里要减一
            }
            if(!Character.isDigit(x) || i== n-1){
                if(sign == '+'){
                    stack.push(num);
                }else if(sign == '-'){
                   stack.push(-1*num); 
                }else if(sign =='*'){
                    stack.push(stack.pop()*num);
                }else if(sign =='/'){
                    stack.push(stack.pop()/num);
                }
                num=0;
                sign=x;
            }
        }
        int sum=0;
        while(!stack.isEmpty()){
            sum+=stack.pop();
        }
        
        return sum;
        
    }
    
    
    
    
    
    
}