#include <bits/stdc++.h>
using namespace std;
const long double pi = 3.14159265359;
inline long long add(long long a, long long b, long long m) {
  if ((a + b) >= m) return (a + b) % m;
  return a + b;
}
inline long long mul(long long a, long long b, long long m) {
  if ((a * b) < m) return a * b;
  return (a * b) % m;
}
long long power(long long x, long long y, long long m) {
  long long res = 1;
  x = x % m;
  if (x == 0) return 0;
  while (y > 0) {
    if (y & 1) res = (res * x) % m;
    y = y >> 1;
    x = (x * x) % m;
  }
  return res;
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
int main() {
  ios_base::sync_with_stdio;
  cin.tie(0);
  cout.tie(0);
  int t = 1, n;
  cin >> t;
  while (t--) {
    string s;
    cin >> n;
    cin >> s;
    string ans;
    int i = 0;
    for (; i < s.length(); i++) {
      if (s[i] == '0') {
        ans.push_back(s[i]);
      } else {
        break;
      }
    }
    string ans2;
    for (int i = s.length() - 1; i > -1; i--) {
      if (s[i] == '1') {
        ans2.push_back(s[i]);
      } else {
        break;
      }
    }
    if (ans.length() + ans2.length() == s.length()) {
      cout << s << endl;
    } else
      cout << ans << "0" << ans2 << endl;
  }
  return 0;
}
