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public class LongestSubstringKUnique {
    public static void findLongestSubstring(String str, int k) {
        if (str == null || str.length() == 0) {
            System.out.println("Invalid input");
            return;
        }

        // Check if there are enough unique characters
        int uniqueCount = (int) str.chars().distinct().count();
        if (uniqueCount < k) {
            System.out.println("Not enough unique characters");
            return;
        }

        // Initialize pointers, data structures, and variables
        int i = 0; // Start pointer of the sliding window
        int j = 0; // End pointer of the sliding window
        int[] charCount = new int[26]; // Frequency array for 'a' to 'z'
        int maxLength = 0; // To store the maximum length of the substring
        int maxStart = 0; // To store the start index of the longest substring

        // Traverse the string using the end pointer (j)
        while (j < str.length()) {
            char c = str.charAt(j);
            // Add the current character to the frequency array
            charCount[c - 'a']++;

            // Shrink the window until there are exactly k unique characters
            while (countUniqueCharacters(charCount) > k) {
                char leftChar = str.charAt(i);
                charCount[leftChar - 'a']--;
                i++; // Move the start pointer forward
            }

            // Update the maximum length if the current window is valid
            if (countUniqueCharacters(charCount) == k && (j - i + 1) > maxLength) {
                maxLength = j - i + 1;
                maxStart = i;
            }

            // Expand the window by moving the end pointer
            j++;
        }

        // Output the result
        System.out.println("Longest substring length: " + maxLength);
        if (maxLength > 0) {
            System.out.println("Substring: " + str.substring(maxStart, maxStart + maxLength));
        }
    }

    // Helper method to count unique characters in the array
    private static int countUniqueCharacters(int[] charCount) {
        int count = 0;
        for (int freq : charCount) {
            if (freq > 0) {
                count++;
            }
        }
        return count;
    }

    public static void main(String[] args) {
        findLongestSubstring("aabbcc", 1); // Output: 2
        findLongestSubstring("aabbcc", 2); // Output: 4
        findLongestSubstring("aabbcc", 3); // Output: 6
        findLongestSubstring("aaabbb", 3); // Output