#include <bits/stdc++.h>
using namespace std;
int main() {
  int t;
  cin >> t;
  while (t--) {
    int n;
    cin >> n;
    string s;
    cin >> s;
    int left0 = 0;
    for (int i = 0; i < n; i++) {
      if (s[i] == '0') {
        left0 = i + 1;
      } else {
        break;
      }
    }
    int right1 = 0;
    for (int i = n - 1; i >= 0; i--) {
      if (s[i] == '1') {
        right1 = right1 + 1;
      } else {
        break;
      }
    }
    for (int i = 0; i < left0; i++) {
      cout << 0;
    }
    if (left0 + right1 == n - 1 && (left0 != 0 || right1 != 0)) {
      cout << s[left0];
    } else if (left0 + right1 != n) {
      cout << 0;
    }
    for (int i = 0; i < right1; i++) {
      cout << 1;
    }
    cout << "\n";
  }
  return 0;
}
