题解 | #四则运算#

import java.util.Scanner;
import java.util.Stack;
import java.io.InputStream;
import java.io.PrintStream;
import java.io.IOException;

public class Main {
    public static void main(String[] args) {
        InputStream inpts=System.in;
        Scanner in = new Scanner(inpts);
        
        String calStr=in.nextLine();
        try{
            inpts.close();
        }catch(IOException e){
            e.printStackTrace();
        }
        in.close();
        
        char c='\u0000';
        int i=0;
        int n=calStr.length();
        int t=0;
        int j=0;
        int nums[][]=new int[n][2];//存放字符串中数字对应的值和索引
        boolean bn1=false;
        for(;i<n;++i){
            c=calStr.charAt(i);
            if((c-'0'|'9'-c)>0){
                t*=10;
                t+=c-48;
                bn1=true;
            }else if(bn1){
                nums[j][0]=t;
                nums[j++][1]=i-1;
                t=0;
                bn1=false;
            }
        }
        nums[j][0]=t;
        nums[j++][1]=i-1;
        float res=calculate(calStr,nums);

        PrintStream pt=System.out;
        pt.println((int)res);
        pt.close();
    }
    public static float calculate(String s,int num[][]){
        Stack<Character> symbol=new Stack<>();
        Stack<Float> number=new Stack<>();
        int i=0;
        int j=0;
        int n=s.length();
        char c='\u0000';
        char cTmp='\u0000';
        float m=0;
        for(;i<n;++i){
            c=s.charAt(i);
            if((c-'0'|'9'-c)>0){
                m=num[j][0];
                number.push(m);
                i=num[j++][1];
            }else if(c=='('||c=='['||c=='{'){
                symbol.push('(');
            }else if(c==')'||c==']'||c=='}'){
                cTmp=symbol.peek();
                while(cTmp!='('){
                    twoNumRes(number,symbol);
                    cTmp=symbol.peek();
                }
                symbol.pop();
            }else{
                cTmp=s.charAt(i-1);
                if(i==0||i>0&&(cTmp=='('||cTmp=='['||cTmp=='{')){
                    number.push(0f);
                }
                while(!symbol.isEmpty()&&symbol.peek()!='('){
                    cTmp=symbol.peek();
                    if(canCalFirst(c,cTmp)){
                        twoNumRes(number,symbol);
                    }else{
                        break;
                    }
                }
                symbol.push(c);
            }
        }

        while(!symbol.isEmpty()){
            twoNumRes(number,symbol);
        }
        return number.peek();
    }
    public static void twoNumRes(Stack<Float> num,Stack<Character> sym){
        float b=num.pop();
        float a=num.pop();
        char cs=sym.pop();
        float res=0f;
        if(cs=='+'){
            res=a+b;
        }else if(cs=='-'){
            res=a-b;
        }else if(cs=='*'){
            res=a*b;
        }else{
            res=a/b;
        }
        num.push(res);
    }
    public static boolean canCalFirst(char c0,char cTop){
        if((c0=='*'||c0=='/')&&(cTop=='+'||cTop=='-')){
            return false;
        }
        return true;
    }
}