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.


def maxUniqueSplit(s, start=0, seen=None):
    if seen is None:
        seen = set()
    if start == len(s):
        return 0
    maxUnique = -1
    for i in range(start + 1, len(s) + 1):
        sub = s[start:i]
        if sub not in seen:
            seen.add(sub)
            unique = maxUniqueSplit(s, i, seen)
            if unique != -1:
                maxUnique = max(maxUnique, unique + 1)
            seen.remove(sub)
    return maxUnique
