// you implement this
//To store result of addition
string add_result="";
//stores the sum of digits
int sum_of_digits=0,len_of_b1,len_of_b2;
len_of_b1=b1.size()-1;
len_of_b2=b2.size()-1;
while(len_of_b1>=0 || len_of_b2>=0 || sum_of_digits==1){
   //find sum of two digits
   if(len_of_b1>=0){
       sum_of_digits+=b1[len_of_b1]-'0';
       }
       else{
           sum_of_digits+=0;
       }

       if(len_of_b2>=0){
       sum_of_digits+=b2[len_of_b2]-'0';
       }
       else{
           sum_of_digits+=0;
       }

       //calculated sum is 3 or 1
       //then ,concatenate the result as
       add_result=char(sum_of_digits%2+'0')+add_result;

       //then find the carry
       sum_of_digits=sum_of_digits/2;
       //then decrement len_of_b1 and len_of_b2
       //to go to next digits
       len_of_b1--;
       len_of_b2--;

   }
return add_result;


}