Im unable to Understand This

#1

Mam can u plz tell me how to approach with ?

#2

see this :

   int convert(string c, int n){    
        if(n== 0){
            return 0;
        }
        int digit = c[n-1] - '0';
        int x = convert(c,n-1);
        return x*10 + digit;
    }