Find error in my code

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;

string find_binary(ll a)
{
string res="";
while(a!=0)
{
res+=(a%2LL+‘0’);
a/=2LL;
}
return res;
}
int main() {

ios::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
ll t,a,b;
string s1,s2;
cin>>t; 
while(t--)
{
    
    ll count=0;
    cin>>a>>b;
    s1=find_binary(a);
    s2=find_binary(b);
    
    if(s1.length()>s2.length())
    {
        while(s2.length()<s1.length())
        {
            s2+='0';
        }
    }
    else if(s2.length()>s1.length())
    {
        while(s1.length()<s2.length())
        {
            s1+='0';
        }
    }
    reverse(s1.begin(),s1.end());
    reverse(s2.begin(),s2.end());
    
    //cout<<s1<<" "<<s2<<endl;
    for(int i=0;i<s1.length();i++)
    {
        if(s1[i]!=s2[i]) count+=1;
    }
    cout<<count<<endl;
}

}

Problem link ?.. .

Here is the problem link :https://www.codechef.com/problems/BINSHFFL
Also please suggest me how can i improve my coding skills (how can i approach a new topic which i don’t know) as iam really struggling hard to improve since a few months.
Thanks in advance.

One thing I found in your code
res+=(a%2+'0'); a/=2;