Given a string `s`, return the maximum number of unique substrings that the given string can be split into.

You can split string `s` into any list of non-empty substrings, where the concatenation of the substrings forms the original string. However, you must split the substrings such that all of them are unique.

A substring is a contiguous sequence of characters within a string.

Example 1:

    Input: s = "ababccc"
    Output: 5
    Explanation: One way to split maximally is ['a', 'b', 'ab', 'c', 'cc']. Splitting like ['a', 'b', 'a', 'b', 'c', 'cc'] is not valid as you have 'a' and 'b' multiple times.

Example 2:

    Input: s = "aba"
    Output: 2
    Explanation: One way to split maximally is ['a', 'ba'].

Example 3:

    Input: s = "aa"
    Output: 1
    Explanation: It is impossible to split the string any further.

Constraints:

- `1 <= s.length <= 16`

- `s` contains only lower case English letters.