Untitled

import java.util.HashSet;

public class Solution {
    public int countGoodSubstrings(String s, int k) {
        int n = s.length();
        if (n < k) return 0;

        int count = 0;
        HashSet<Character> window = new HashSet<>();

        for (int i = 0; i < n; i++) {
            // Add the current character to the window
            window.add(s.charAt(i));

            // Remove the character that slides out of the window
            if (i >= k) {
                window.remove(s.charAt(i - k));
            }

            // Check if the window size equals k and all characters are unique
            if (window.size() == k) {
                count++;
            }
        }

        return count;
    }
}