2GIS

#include <iostream>
using namespace std;
int main() {
    string  str="15?+15?=30?",
            str_cpu=str;
    int flag=0;
    int question='?'-'0';
    int a=0,b=0,c=0,d=0;
    int     posPlus=-1,
            posEq=-1;
    // Определение позиций знаков '=' и '+'
    for (int i=0;i<str.size();i++){
        switch(str[i]){
            case '+': posPlus=i; break;
            case '=': posEq=i;  break;
            default: break;
        }
    }
    for (int pos=0;pos<posPlus;pos++){
        a=str[posPlus-pos-1]-'0';
        b=str[posEq-pos-1]-'0';
        c=str[str.size()-pos-1]-'0';
        // 1 неизвестная
        if((a==question)&&(b!=question)&&(c!=question)){
            a=c-b-d;
            if(a<0) a=a+10;
        }
        else if((a!=question)&&(b==question)&&(c!=question)){
            b=c-a-d;
            if(b<0) b=b+10;
        }
        else if((a!=question)&&(b!=question)&&(c==question)){
            c=(a+b+d)%10;
        }
            //2 неизвестных
        else if((a==question)&&(b==question)&&(c!=question)){
            if(flag==0){
                a=c;
                b=0;
            }
            else if(flag==1){
                a=c-5;
                b=5;
            }
        }
        else if((a!=question)&&(b==question)&&(c==question)){
            if(flag==0){
                c=a;
                b=0;
            }
            else if(flag==1){
                b=10-a;
                c=(a+b+d)%10;
            }
        }
        else if((a==question)&&(b!=question)&&(c==question)){
            if(flag==0){
                c=b;
                a=0;
            }
            else if(flag==1){
                a=10-b;
                c=(a+b+d)%10;
            }
        }
            //3 неизвестных
        else if((a==question)&&(b==question)&&(c==question)){
            if(flag==0){
                a=0;
                b=0;
                c=0;
            }
            else if(flag==1){

                a=5-d;
                b=5;
                c=0;
            }
        }
        cout<<"pos="<<pos<<" a="<<a<<" b="<<b<<" c="<<c<<" d="<<d;
        if(c==(a+b+d)%10) {
            cout << " flag=" << flag << " good " << endl;
            str_cpu[posPlus-pos-1]=(char)a+'0';
            str_cpu[posEq-pos-1]=(char)b+'0';
            str_cpu[str.size()-pos-1]=(char)c+'0';
            //if(flag) flag=0;
        }
        else{
            cout<<" flag="<<flag<<" impossible step"<<endl;
            if(!flag){
                cout<<"change flag"<<endl;
                flag=1;
                pos=pos-2;
            }
            else{
                cout<<"impossible";
                return 0;
            }
        }
        d=(a+b+d)/10;
    }
    cout<<str_cpu;
    return 0;
}