算数(从左往右依次计算)

#include <bits/stdc++.h>
using namespace std;
int main(){
    string s;
    getline(cin,s);
    char x;
    int a,a1,s1 = s.size();
    bool have = true;
    for(int i = 0;i<s1;i++){
        if((s[i]=='+' || s[i]=='-' || s[i]=='*' || s[i]=='/') && have){
            x = s[i];
            a = stoi(s.substr(0,i));
            s = s.substr(i+1);
            have = false;
        }
        if(have){
            continue;
        }
        int j = 0;
        while(1){
            if(s[j]=='+' || s[j]=='-' || s[j]=='*' || s[j]=='/' || j>=s.size()-1){
                break;
            }else{
                j++;
            }
        }
        a1 = stoi(s.substr(0,j+1));
        if(x=='+'){
            a += a1;
        }else if(x=='-'){
            a -= a1;
        }else if(x=='*'){
            a *= a1;
        }else if(x=='/'){
            a /= a1;
        }
        x = s[j];
        if(s.size()<=j+1){
            break;
        }
        s = s.substr(j+1);
    }
    cout<<a;
    return 0;
}