class Solution {
    public int lengthOfLongestSubstringKDistinct(String s, int k) {

        int len_max = 0;
        int count_k = 0;
        int i_left = 0;
        int i_right = -1;

        // int[] char_count = new int[26];
        Map<Character, Integer> char_count = new HashMap<>();

        while (true) {
            if (i_left == s.length()) break;

            if (count_k <= k) {
                // found one possible answer
                int len_now = i_right - i_left + 1;
                len_max = Math.max(len_max, len_now);

                // still have room to grow length
                i_right++;
                if (i_right == s.length()) break;

                char c_now = s.charAt(i_right);
                // int i_now = c_now - 'a'; // all char is lower-case?
                if (char_count.get(c_now) == null || char_count.get(c_now) == 0) {
                    // a new char inserted
                    char_count.put(c_now, 0);
                    count_k++;
                }
                char_count.put(c_now, char_count.get(c_now) + 1);

            }else {
                // one k too much, try to reduce it

                char c_now = s.charAt(i_left);
                char_count.put(c_now, char_count.get(c_now) - 1);
                if (char_count.get(c_now) == 0) {
                    // the last char here is removed
                    count_k--;
                }
                i_left++;
            }

        }

        return len_max;
    }
}