#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007;
const int inf = 1e9 + 10;
const long long INF = 1e18;
const long double EPS = 1e-10;
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout << fixed << setprecision(25);
  int t;
  cin >> t;
  while (t--) {
    int n;
    string s;
    cin >> n >> s;
    string ans = "";
    int idx = 0;
    while (idx < n && s[idx] == '0') {
      ans += s[idx];
      idx++;
    }
    s = s.substr(idx);
    reverse(s.begin(), s.end());
    n = s.size();
    string ans2 = "";
    idx = 0;
    while (idx < n && s[idx] == '1') {
      ans2 += s[idx];
      idx++;
    }
    s = s.substr(idx);
    n = s.size();
    if (n) ans += '0';
    ans += ans2;
    cout << ans << '\n';
  }
  return 0;
}
