Split a String Into the Max Number of Unique Substrings

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.

Solution:

class Solution {
    int max = 1;
    
    public int maxUniqueSplit(String s) {
        helper(s, 0, new StringBuilder(), new HashSet());
        return max;
    }
    
    private void helper(String s, int i, StringBuilder curr, Set<String> set) {
        if (i == s.length()) {
            if (curr.length() > 0 && set.add(curr.toString())) {
                max = Math.max(max, set.size());
                // System.out.println(set);
                set.remove(curr.toString());
            }
            return;
        }
        char c = s.charAt(i);
        curr.append(c);
        if (set.add(curr.toString())) {
            helper(s, i + 1, new StringBuilder(), set);
            set.remove(curr.toString());
        }
        helper(s, i + 1, curr, set);        
    }
}