#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t;
  cin >> t;
  while (t--) {
    long long int n, z = 0, o = 0;
    cin >> n;
    string s;
    cin >> s;
    if (n == 1) {
      cout << s << endl;
      continue;
    }
    for (long long int i = 0; i < n; i++) {
      if (s[i] == '0')
        z++;
      else
        break;
    }
    z++;
    for (long long int i = n - 1; i >= 0; i--) {
      if (s[i] == '1')
        o++;
      else
        break;
    }
    if (o + z == n + 1) {
      cout << s << endl;
      continue;
    }
    while (z) {
      cout << '0';
      z--;
    }
    while (o) {
      cout << '1';
      o--;
    }
    cout << endl;
  }
}
