340. Longest Substring with At Most K Distinct Characters

class Solution {
    public int lengthOfLongestSubstringKDistinct(String s, int k) {
        int n = s.length(), i = 0, re = 0;
        Map<Character, Integer> map = new HashMap<>();
        for(int j = 0; j < n; j++)
        {
            char ch = s.charAt(j);
            map.put(ch, map.getOrDefault(ch, 0) + 1);
            while(map.keySet().size() > k)
            {
                char rem = s.charAt(i++);
                map.put(rem, map.get(rem) - 1);
                if(map.get(rem) == 0) map.remove(rem);
            }
            re = Math.max(re, j + 1 - i);
        }
        return re;
    }
}