First non-repeating character in a stream

//{ Driver Code Starts
#include<bits/stdc++.h>
using namespace std;

// } Driver Code Ends
class Solution {
    public:
        string FirstNonRepeating(string A){
            // Code here
           queue<int> q;
           q.push(A[0]);
           unordered_map<char,int> m;
           m[A[0]]++;
           int i=1;
           string ans="";
           ans+=A[0];
           while(i<A.length())
           {
               m[A[i]]++;
               q.push(A[i]);
               while(m[q.front()]>1 && !q.empty())
               {
                   q.pop();
               }
               if(!q.empty())
               {
                   ans+=q.front();
               }
               else
               {
                   ans+='#';
               }
               i++;
           }
           return ans;

        }

};

//{ Driver Code Starts.
int main(){
    int tc;
    cin >> tc;
    while(tc--){
        string A;
        cin >> A;
        Solution obj;
        string ans = obj.FirstNonRepeating(A);
        cout << ans << "\n";
    }
    return 0;
}
// } Driver Code Ends