Longest Substring with At Most K Distinct Characters

Given a string s, find the length of the longest substring T that contains at most k distinct characters.

Example

For example, Given s = "eceba", k = 3,

T is "eceb" which its length is 4.

Notes:

1. Forward two pointers
2. inner loop: size to k (break when size == k and !map.containksKey(c))
3. template: out loop -> inner loop -> max -> remove i

    public int lengthOfLongestSubstringKDistinct(String s, int k) {
        if (s == null || s.isEmpty() || k <= 0) {
            return 0;
        }

        int n = s.length();
        int j = 0;
        int max = 0;
        Map<Character, Integer> map = new HashMap<>();

        for (int i = 0; i < n; ++i) {
            while(j < n) {
                char c = s.charAt(j);
                if (map.containsKey(c)) {
                    map.put(c, map.get(c)+1);
                } else {
                    if (map.size() == k) {
                        break;
                    }    
                    map.put(c, 1);
                }

                j++;
            }

            max = Math.max(max, j-i);

            char c = s.charAt(i);
            int num = map.get(c);
            if (num == 1) {
                map.remove(c);
            } else {
                map.put(c, num-1);
            }
        }

        return max;
    }