Given an array of strings `nums` containing `n` unique binary strings each of length `n`, return a binary string of length `n` that does not appear in `nums`. If there are multiple answers, you may return any of them.

Example 1:

    Input: nums = ["01","10"]
    Output: "11"
    Explanation: "11" does not appear in nums. "00" would also be correct.

Example 2:

    Input: nums = ["00","01"]
    Output: "11"
    Explanation: "11" does not appear in nums. "10" would also be correct.

Example 3:

    Input: nums = ["111","011","001"]
    Output: "101"
    Explanation: "101" does not appear in nums. "000", "010", "100", and "110" would also be correct.

Constraints:

- `n == nums.length`
- `1 <= n <= 16`
- `nums[i].length == n`
- `nums[i]` is either `'0'` or `'1'`.
- All the strings of `nums` are unique.