#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
  int tc;
  cin >> tc;
  while (tc--) {
    int n;
    cin >> n >> ws;
    string str;
    cin >> str;
    int flag = 0, i;
    for (i = 0; i < str.size() - 1; i++) {
      if (str[i] > str[i + 1]) {
        flag = 1;
        break;
      }
    }
    if (flag == 0) {
      cout << str << endl;
      continue;
    }
    string clean_str;
    for (i = 0; i < str.size(); i++) {
      if (str[i] == '1') break;
      clean_str.push_back('0');
    }
    clean_str.push_back('0');
    for (i = str.size() - 1; i >= 0; i--) {
      if (str[i] == '0') break;
      clean_str.push_back('1');
    }
    cout << clean_str << endl;
  }
  return 0;
}
