The first thing to note is that the length of B doesn’t matter. We only need the unique characters and they are at max 26. Just create a set consisting of all unique letters of B. now iterate over A, if the element is not present in the set, then add it to the resultant string.
#include <bits/stdc++.h>
using namespace std;
int main()
{
int t;
cin>>t;
while(t--){
string a,b;
cin>>a>>b;
set<char>s1;
// As explained, we only need unique values of b, which I am doing by making a set
for(auto it:b){
s1.insert(it);
}
// create a resultant string
string res="";
// iterate over all characters of a
for(auto it:a){
// Check if the character is present in the set
if(s1.find(it)==s1.end()){
// if not present then add it to resultant array
res+=it;
}
}
cout<<res<<endl;
}
return 0;
}