Number of Ways to Split a String

Given a string s with length n, how many ways can you split it into two substrings s_1 and s_2 such that the number of unique characters in s_1 and s_2 are the same?

Parameter

• s: A string with length n.

Result

• The number of ways you can split it into two substrings that satisfy the condition.

Examples

Example 1:

Input: s = "aaa"

Output: 2

Explanation: It can be split in two ways: "a", "aa" and "aa", "a".

Example 2:

Input: s = "bac"

Output: 0

Explanation: There is no way to split this string into two substrings with equal unique elements.

Constraints

• 0 <= n <= 10^5

• Characters in this string may consist of any alphanumeric character. The characters are case sensitive. That is, same letters with different cases count as different characters.

public class Solution
{
    public int CountWaysToSplit(string s)
    {
        int ways = 0;
        if (s?.Length > 0)
        {


            int[] prefixCount = new int[26];
            int[] suffixCount = new int[26];
            int prefixUnique = 0;
            int suffixUnique = 0;


            foreach (char c in s)
            {
                if (suffixCount[c - 'a']++ == 0)
                {
                    suffixUnique++;
                }
            }

            for (int i = 0; i < s.Length - 1; i++)
            {
                if (prefixCount[s[i] - 'a']++ == 0)
                {
                    prefixUnique++;
                }
                if (--suffixCount[s[i] - 'a'] == 0)
                {
                    suffixUnique--;
                }
                if (prefixUnique == suffixUnique)
                {
                    ways++;
                }
            }
        }

        return ways;
    }
}